Hello, today I changed my nVidia card for a Vega 64. When I start RaceRoom, I can't choose a higher resolution than 1280x768. I have no issues with other games. Right now I'm trying to validate my steam files. That didn't help.
Have you tried Display Driver Uninstaller, use it to remove all traces of both AMD and Nvidia drivers and fresh install, also try deleteing graphics_options.xml in \Documents\My Games\SimBin\RaceRoom Racing Experience\UserData and try the game then.
I have to ask, did you change the aspect ratio before trying to choose a resolution? When a config change is detected (graphics wise) this setting is reset and restricts the options available until you change it to say 16:9 (most common) I only know because it does it every time I switch between vr and triples (wish there was a separate config file for vr)