Hi Bastien,
just a little thought because I had an issue like this (but with cyrus):
Where is your thunderbird installation stored?
In my case, it was not locally but on some windows mapped drive, and so also the imap cache of thunderbird was on this drive. Because we have a lot of mails in our accounts, the cache was about 50-100MB for some users, and users with a lot mail accounts had very often timeouts. This was due to performance problems on our network storage. Sometimes it also works to delete thunderbird´s cache files. Then it has to download all the headers again (bad with bad performing network storage :-/), but it seems that these cache files get corrupted from time to time and this also produces timeouts.
kind regards
Andreas
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