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You're the first to report it, so it seems it is not a common problem. I've updated tons of websites to 2.5.6 and didn't encounter this issue. If the avatars were working on XOOPS 2.5.5, there should be no problem with XOOPS 2.5.6
Was this English version of Dutch version?
If you can, please set up a copy of your current installation on a subdirectory on subdomain, upgrade to XOOPS 2.5.6, and if it still doesn't work, let me take a look into it, so I can compare your 2.5.5 installation and compare it to your upgraded version to see what is happening there