?Why do you think that wp fastest cache causes the issue?
As the issue started after installtion of the plugin, but stayed after deinstallation i thought some redirects were installed or simila.
Compared .htaccess with a backup, no changes.
But what worked:
Changed WP into maintenance Mode, reloaded page on mobile device, showed maintenance page.
Changed back to WordPress: Side loading now again.
Worked with 2 devices. But I dont have an explanation why.