OK, thanks for your reply.
I managed to get the site live again.
A modesecurity rule had been triggered on my webserver:
“Detects MySQL comment-/space-obfuscated injections and backtick termination”
It thinks the site is being attacked with sql injection.
As a temporary fix I disable Modesecurity and the site went live again.
Before I broke it again on the mobile site I deactivated the plugin and that didn’t help.
