Can you try switching to a different skin temporarily, to see if add-ons are then accessible? If this is the case, we can reinstall the skin's files/config and hopefully fix the issue.
You don't happen to have downloaded any music, videos or add-ons with accents/umlauts in them recently have you? à é ü ň etc.
[edit]
Sorry, I should have added, to switch to a different skin, open XBMC then:
System>Appearance>Skin>Skin
Make a note of the current skin chosen, then click on a different one.
Restart XBMC, make sure a new skin is now in use, and try to access the add-ons again.
By the way, can you see if you're able to access
Pictures>Add-ons
or not.