Frontend upload error

On a different server i cannot upload files from frontend. Upload failed with error:
Error: XHR returned response code 500
It works in the backend.
Any idea?

Thanks

The console says:

TypeError: i.fire is not a function[Weitere Informationen] tinymce.min.js:11:24486
[Fine Uploader 5.15.4] GET request for sessionRefresh has failed - response code 500 fine-uploader.min.js:2:2560
[Fine Uploader 5.15.4] Problem parsing session response: JSON.parse: unexpected end of data at line 1 column 1 of the JSON data fine-uploader.min.js:2:2560
[Fine Uploader 5.15.4] Caught exception in 'onSessionRequestComplete' callback - response is null fine-uploader.min.js:2:2560
[Fine Uploader 5.15.4] Error when attempting to parse xhr response text (JSON.parse: unexpected end of data at line 1 column 1 of the JSON data) fine-uploader.min.js:2:2560
Source-Map-Fehler: request failed with status 404
Ressourcen-Adresse: https://wiesbauer-marketing.de/administrator/components/com_showtime/js/vendor/fine-uploader.min.js?1.3.0
Source-Map-Adresse: fine-uploader.min.js.map[Weitere Informationen]

Thank you Lefteris!