So if the computer is shutdown during a nbackup the database is locked in this way indefinelty?

Yes, it's one of the most prevalent problems with nbackup.
To prevent it you need to monitor delta file age and size - manually, or automatically using our FBDataGuard tool.

What do you suggest I should do with JayBird when the exception is thrown?
Execute "alter database end backup"?

Yes, it should merge delta file back to the main database and unlock it.

Regards,
Alexey Kovyazin
IBSurgeon