I found a discussion about this problem where many people were angry at Google because they don't fix this problem.
I found out that the problem was due to my browser security plugins Adblock Plus and NoScript. At least one of those (I think it was NoScript) was blocking the site gstatic.com (and google.com of course or at least maps.google.com). No need to completely disable (which was mentioned also in the discussion), just whitelist relevant sites in those plugins (Check site media information to see loaded images for example).
Some other people mentioned different reasons causing the same problem like:
- Check your orther privacy or security plugins for blocking maps.google.com or gstatic.com
- Delete cookies for maps.google.com as well as all temporary internet files (clear cache).
- Skype extension for Firefox causing the problem.
- DNS problems (that usually temporarily - to separate this reason from others check availability of maps.google.com and gstatic.com using nslookup and check if it works using a different browser without any plugins - if both works then DNS is not the reason for your problem).
- if2k (if on Mac).
- Badly written router firewalls (try deactivating IPv6).
- Turn off inprivate filtering (if on Windows IE).
- Windows only: Uninstall Java and reinstall launching as Administrator (Win 7).
- Your mobile web service provider might have implemented an accelerator reducing image quality - check and turn off on the provider's website.
I am posting this because there were many responses to this and still some are posting without reading the earlier answers offering solutions and it seems that plenty of people experiencing problems with google maps.