I solved it with the help of this post. I needed to remove the trailing slashes from the ProxyPass directives.
becomes
ProxyPass "/nextcloud" "http://127.0.0.1:8000" # Snap instance
ProxyPassReverse "/nextcloud" "http://127.0.0.1:8000"
ProxyPass "/oldcloud" "http://192.168.1.11/oldcloud" # Not snap
ProxyPassReverse "/oldcloud" "http://192.168.1.11/oldcloud"
ProxyPass "/" "http://127.0.0.1:8001" # Static files
ProxyPassReverse "/" "http://127.0.0.1:8001"