I use Thunderbird 115.8.0 (64-bit) on Windows to check 10 email accounts all on the same server. The setup in Tbird for all accounts is identical. I only have a problem with one account. The problem is that on one account, after a small number of replies on a thread, the Send button becomes greyed out and emails will not save. The other email accounts continue to work just fine. Compacting mailboxes does not fix the issue.