Hey Olollo,
the 500 error might be caused by numerous reasons, and is in general due to the workings of the server (as the error message suggests). One thing to try before you contact your hosting provider though is that the file you’re uploading as the demo content has the .xml extension and not any other. Another possible culprit, on the server side already, is the maximum upload size limitation set by your hosting provider, but normally they shouldn’t get in the way since the demo content file is quite small by any standard (less than half a megabyte).
Sincerely,
Andrii / Satori Studio