I used to be in an arma 3 unit and it would always take forever to load into a server lobby. In RCon I would always get stuck on verifying GUID.
I left the unit and the issue stopped but it never really occurred to me that was the issue until I joined another unit and started having the problem again.
It only happens if you have a unit selected from your launcher and it isn't 100% of time time that it happens. Once it starts it will not stop until you disable the unit and/or delete the .arma3profile you're using. Deleting the profile is only a temp fix though obviously.
I'm sorry I cant give any more details, this is all I have been able to figure out.