This is what Steam says about this issue:
Please open Internet Explorer, Safari or Firefox and type
steam://flushconfig then press Enter. (If asked to allow this website
to open a program, please click 'Allow' or 'OK')
When prompted by Steam to reset your configuration, please click 'OK'
Once done, login to Steam and re-test the issue.
If you have multiple installation folders for games doing this process
will remove them. You will need to add the folders again in Steam so
that your games will appear as installed.
In order to add the folders please do the following:
- Open Steam
- Click Steam > Settings (Preferences on Mac)
- Navigate to the 'Downloads' tab
- Click the 'Steam Library Folders' button
- Click on 'Add Library Folder' and add the directory that contains your games
- Click 'OK'
- Restart Steam
If the issue persists, make sure your network is optimized for Steam. Check the network troubleshooting support page, especially the ports section. Also some ISPs have trouble with Steam:
- 012.net (Steam traffic blocked)
- Bluewin (Firewall blocks Steam from provider side - must be disabled through provider's service portal)
- Dutch Telecom (Steam traffic blocked)
- ISPFree (Steam traffic blocked)
- Micronet Broadband (Steam traffic blocked)