I've tried linux-next today (4.6.0-rc3-next-20160412) as I've read there are quite some intel driver changes there and this issue with freezing is gone. I can now log-in with monitor attached and everything still works.
The message that we posted here still appears thus confirming what Matt said in comment 35 that it doesn't have a thing to do with our reported problems.
I've tried linux-next today (4.6.0- rc3-next- 20160412) as I've read there are quite some intel driver changes there and this issue with freezing is gone. I can now log-in with monitor attached and everything still works.
The message that we posted here still appears thus confirming what Matt said in comment 35 that it doesn't have a thing to do with our reported problems.