[Typo3] extensions uninstalling themselves?

[Pieter Jelle]

Hello all

I just created a new site with Typo3, in which I use some extensions 
(tt_news and a few others). But, in the last few days, some extensions 
uninstalled themselves. It has happened with tt_news, jh_random_records 
and a small extension I developed some time ago (which has been used in 
other installations without any problem).
It seems they have disappeared from the list of loaded extensions, and 
they're no longer functional on the site (although no error message 
appears). I can get them back to work by clicking the import/update 
button and clearing the cache, or uploading the t3x file again and 
overwriting the extension.
Has anyone got an idea on what could be the cause of this? Thanks in 
advance for any advice!

Best regards
Pieter Jelle