So, this is my setup : xUbuntu 18.04 Nvidia 390.48 on Nvidia TI750 card Dual Monitor (one on DispayPort, Second is DVI) Multiple users.
My problem is - when i boot - everything is fine and work as expected - desctop is extended on both monitors. When i log in as first user - its still OK. BUT, once i try to change user shit happens. GDM screen first drops on VGA resolution (640x480). Second user logged in have both screen mirrored (not extended) and both in VGA mode. Nvidia settings started by second user shows basically nothing. No info about driver, about card, anything. If i simply change back to first user - everything go back fine! Again. If i log out both users and relogin second user - it will work on full res.
To cut it short - only one session works normally. I have tried newer, Nvidia 396 driver from official PPA - no good either. Basically same problem and frequent xorg crashes. 390.77 from same PPA - no changes.
On 17.04 and Nvidia 384 i have no such problem.
I have searched internet and found that some peoples have similar problem, its related to dual monitor setup it seems.
Its also seems that one of 390 drivers had such problem according to Nvidia release notes, here it is - https://www.nvidia.com/download/driverResults.aspx/130646/en-us
"Fixed a regression introduced in 390.12 that prevented displays from working normally when running multiple X screens with emulated overlays."
Sounds very similar to my problem, but its should be fixed already.
This problem is blocker for 18.04 to me.
Its finally seems that second monitor have nothing to do with problem. Either disabling second monitor in setting or even physically disconnecting it from system changes nothing. Now i completely dont understand that source of problem is.