Nextcloud does not accept all file suffixes...is there any solution?

File Access Control and ransomware apps are the only thing I could imagine to block this in Nextcloud.

You can check your webserver, perhaps the configuration does not allow .inc-files. For apache it could look like this:

<Files  ~ "\.inc$">
  Order allow,deny
  Deny from all
</Files>

Can you try on the demo instances, perhaps it is a client issue: