Alright this is a big puzzle so let me get to it.  First let me state that I have the base game, 3 DLC's and the Merc expansion. I have GC3 installed on 2 PC's and a laptop which have all played it in the past and had existing game directories.  I fired up the game  2 or 3 weeks ago on my HTPC. I updated to ver 4.2 and I had no issues whatsoever but noticed I wasn't getting credit towards Honorary Stardockian achievement and after perusing the forums decided to roll back to ver 2.8 .  This worked as intended and I got the achievement.  In the course of so doing I switched to my main PC for the higher resolution display.  After reaching the milestone and a few others I realized I'd need to return to 4.2 to enable some other achievements that were added to the game later.  My main PC was busy running some other stuff so I went back to the HTPC and updated the game to 4.2 again.  It will launch but immediately goes into offline mode and thinks I'm not connected to steam which is running just fine.  Uninstalled and reinstalled 3 times using the listed procedure including renaming the MyGames and appdata files and/or just deleting them as well. No luck.  Uninstalled and reinstalled Steam which had no effect either.   My main PC which still has ver 2.8 on it works fine but I'm hesitant to upgrade it for fear of it going the way of the HTPC.  SoaSE Rebellion works just fine from the launcher on all machines.  I'll try again but this has been a huge PITA for hours now. 

- Offline issue persists with HTPC.  I see others posting with the offline problem. Is there another location for an obscure file? I saw some old tachyon files in another directory but I think they're orphans from original install as they're all dated 2017. Maybe a registry edit is req'd?