I install for example qtype_wordselect; no problem during installation and the question type works but if i run "check for available updates" an cURL appear.
The strange thing is the if i delete an other the question type (for example type_regexp) leaving the qtype_wordselect installed then the system return to works, I mean no cURL at "check for available updates".
It seems that the error is linked to the number of plugins or type plugins at least. That's why i thought the error is somehow linked to php and/or DB (dimension, ....).
By the way, there's no proxy or reverse proxy in the network of the ubuntu server but there's a firewall.
Thanks very much for your help Marcus