At this point I can only speculate. A gretl bug seems unlikely since
no one else has had this problem. OTOH, I guess you are running a
Greek (so, non-latin) system, and maybe there's a 1% chance that
there is some weird interaction triggering the problem.
(Any other Greek users out there with or without this problem?)
My bet would still be installation corruption. If you have the
necessary admin rights, perhaps try to remove any gretl-related
directories/folders that remain even after an uninstall (including
the ...Appdata/Roaming... location, if you know what I mean). Also,
re-start Windows before re-installing gretl.