This is an archive post. It will not be updated.
When this error message appears in Contao 4, in my experience it has one of two causes.
Variant 1:
The web space is full (always look at the limit of the normal web space files in combination with emails and databases). The only thing that helps is cleaning up the web space and switching to a package with more storage space.
Variant 2:
It may be that the Contao folders are not installed correctly. The Contao console command contao:install will help.