Eventually this error happens with some files, would it be any restrictions for executable files?
503 is a internal server error. So your webserver logs should help you. You can restrict the file types with the files-access-control app but it is also possible that this is implemented on the webserver directly.
Thank you very much for the answer, it already gave me a way, I will check.
maybe those threads are connected?
This problem is solved :
Synchronization error for Nextcloud Desktop Client
Porém este continua :