I have enough opponents, all were loaded at startup but no fights are being done and no error message is shown
When did you last train your database? If it was less than 24h ago and you try to run fights on the same database then BC won't do fights that were previously successfully done. To avoid this you should always run fights on a copy of your databases. Pupils that were created less than 24h ago will never be trained by BC even if you run fights on a copy of your database.