Yesterday I did an "alternative" test: in the webmail under SETTINGS -> CONNECTIVITY -> MAILBOX MIGRATION instead of using the "Microsoft Exchange" option I used the "SmarterMail" option.
With this option I managed to connect to the OVH Exchchange server that hosts my mail and I managed to do the migration (although I'm not sure that it migrated everything, I still have to check).
If instead I use the "Microsoft Exchange" option it still doesn't work.
For completeness, I am using SM 7417
UPDATE: it seems that not all the mailbox has been migrated.
Exchange OVH reports that the space occupied by my inbox is around 17.2 GB (I also tried to do a exposure from Outlook on PST file and the resulting file is around 19GB), while SmarterMail instead gives the size of the box migrated is approximately 8.7 GB.
From this I deduce that despite the migration process done by SmarterMail SEEMS successful, in reality there is something wrong and that a lot of data is lost on the way.