Update June 2013: It seems this site is still going strong and still helping people out. However I will no longer be updating the numbers below. The numbers you see on this page are a snapshot of September 2012.
How to fix it
Various solutions have been discovered, and these are listed below. As you can see from the responses I've had, every one of them has some chance of solving the problem. The simplest (and most convenient) solutions come first, so I recommend working through them from top to bottom. If none of these work or you have useful information that ought to be added to this page, please tell me.
First, check your available space (Settings -> SD Card & phone storage). If space is low, solutions (e) or (l) will probably work for you. The problem may re-occur if your space runs out again though - consider investing in a better memory card!
a) Restart your phone.[50 people told me this worked]
Restart your phone by switching it off and switching it back on again (not just the screen - you need to shut it down!).
b) Make sure your phone is not on a Wi-Fi network.[32 people told me this worked]
Some Wi-Fi networks may block port 5228, used by the Android Market. If your phone is connecting to the Market via a Wi-Fi network, turn it off (Settings -> Wireless & networks -> Turn off Wi-Fi) and your phone will then connect using APN (i.e. internet access via the mobile network). Alternatively, if you have admin access to the network, open port 5228 (UDP and TCP). Then return to the Market and restart your downloads.
c) Log into Google Talk.[26 people told me this worked]
There should be a Google Talk application already installed on your phone, which will log you in as soon as you load it up. Do this, and then go back to the Market and restart your downloads. If you can't log into Google Talk, try connecting to a wireless network (Settings -> Wireless -> Turn on Wi-Fi). If you still can't log in, the Wi-Fi network may have a firewall (see above), in which case you will need to find a less restrictive Wi-Fi network. Once logged into Google Talk, you can turn off Wi-Fi to switch back to APN connections.
d) Refresh your data syncronization settings.[165 people told me this worked]
Try disabling your application syncronization setting and then re-enabling it (Settings -> Accounts & Sync -> Background data). Uncheck the box and then recheck it. Then restart your downloads.
e) Clear your application data.[34 people told me this worked]
Try clearing up your application data and caches (Settings -> Applications -> Manage Applications). Ensure they are all listed (Menu -> Filter -> All), then go into each of your applications and tap "clear data" or "clear cache". There will be a lot of apps listed, but you probably won't have to clear caches for all of them - do it for as many as you can, particularly the ones you recognise. Caches may be taking up too much storage space, preventing new apps from being downloaded. After clearing caches, restart your downloads.
f) Uninstall the Market update.[84 people told me this worked]
Many users have found that the problem started after installing a Market update. Try uninstalling it (Settings -> Applications -> Manage Applications -> Market). Tap "clear cache" and then "uninstall updates". Then return to the Market, sign in, and restart your downloads. (Don't worry about losing functionality - the update will reinstall, hopefully without the problem returning).
g) Uninstall the Download Manager update.[17 people told me this worked]
Many users have found that the problem started after installing a Download Manager update. Try uninstalling it (Settings -> Applications -> Manage Applications -> Download Manager). Tap "clear data" and then "uninstall updates" (my phone doesn't have this option - and lists the version as version 7. Please let me know if your phone is different). Then return to the Market and restart your downloads.
h) Disable Data Saver.[21 people told me this worked]
Find the Data Saver (Settings -> Battery & Data Saver) and uncheck "Enable Data Saver". Then return to the Market and restart your downloads.
i) Upgrade your phone software.[3 people told me this worked]
Connect your phone to a computer and upgrade your phone's software. You may need to install a PC companion to do so. Then disconnect and restart your downloads.
j) Change your google account password.[4 people told me this worked]
Change your password using a computer. Your phone will ask for the new password when you reload the Market. Then restart your downloads.
k) Switch your googlemail account to a gmail account.[8 people told me this worked]
Log into your googlemail account on a computer. Click on "Settings", then "Accounts and Import" and find the option to switch your address from "@googlemail.com" to "@gmail.com". Then use this account for the Market instead. You may need to reset your phone to link it to the new account format (Settings -> Privacy -> Factory data reset). Annoyingly, if you sign up for an account through your phone, you will be forced to create a googlemail account. But reports indicate that the Market has trouble communicating with googlemail accounts. Switching your account to a gmail account, or using a gmail account you already own, may solve the problem.
l) Uninstall some apps.[2 people told me this worked]
Apps may stop downloading if your phone runs out of space. Try uninstalling some apps and then restart your downloads.
m) Restore your phone to a previous restore point.[2 people told me this worked]
If your phone has software which creates restore points (such as "nandroid"), try restoring your phone to a past state wherein the Market did work. Then try downloading again from the Market.
n) Reset your phone.[39 people told me this worked]
Try resetting your phone (Settings -> Privacy -> Factory data reset). Then try the Market again with your existing Google Account. If this is an undesirable solution, consider the other solutions at the bottom of this page first.
o) Create another Google Account.[15 people told me this worked]
Try creating a new account. You may need to reset your phone to do this (see above). You can either create the account on a computer, or create the account on your phone (both methods have been reported to work). This solution worked for me.
Some other solutions that have worked for only one person:
- Correct the date on the device, if it's set to the wrong date. "Market will not work if date is too far off."
- Correct the timezone on the device, if it's set to the wrong timezone.
- Cancel the automatic updates and manually update instead.
- Follow the long and complicated instructions in the 7th October 2011 post here.
- Download Archtools (from Appslib) and "install Market version 3.3.3(ish)".
- "Clear the downloads log from the browser menu"
- "Disabled IPV6 on my router."
- "Reset my APN settings to default"
- "Set the DNS in my Draytek router using this source"
- "I logged out of my [gmail?] account on my computer"