|great news !!
i was able to fix it !
like you said, it was probably the inbox.msf that was corrupted
i have a folder Thunderbird/Profiles/(defaultuser)/mail/in.telenet.be
(telenet is my provider)
an also a folder Thunderbird/Profiles/(defaultuser)/mail/local folders
it was the file inbox.msf in the "local folders" folder that was corrupted apparently
i deleted it, restarted, the file got rebuilt, and now everything works fine
the inbox file (without extension) in the local folders file got reduced from 1,18 GB to 39 MB
but in the "in.telenet.be" folder, i still have a inbox (also the file without extension) of 728 MB
is that normal ?
already many thanks for the advice !! finally i can use my mail again properly :-)