potato2014

It's not the ticking that causes it. It's the actual process of just going into that advanced menu. Recreate the GPO without going into Advanced and see if it works.

When setting the Group Policy did you go into the Advanced part of the proxy settings? (to make sure all protocols use the same port and to add local exclusions) If so, that is likely your problem.

The correct way (correct me if I'm wrong) is to migrate the mailbox back to on-premise and delete it the usual way.

Why not try proxy activation with the VAMT tool from Microsoft?

The GPO is applied at the domain root BTW. PS: does anyone have problems typing only on edugeek? it seems to skip letters a LOT?

Strange, on my Server 2012 box the normal settings work. (See image). imgur: the simple image sharer