Login does not work for all Browser / Clients


i recently set up a site with the login Plugin to keep all pages protected.
the site is configured as virtual host via a proxyPass directive (Apache 2.2)
and nearly everything works fine.

However - for some clients / browsers the site remains unaccessable with a message ‘Access denied…’ from the plugin module.
this mostly happens on IE oder Edge browsers. Unfortunately i can’t figure out how to solve the problem or even find out why this happens.

could anyone give me some hints on how to investigate this?

Thanks and regards