Hello @Peter Volz ,
Welcome to Microsoft Q&A forum.
Here are some clues:
The OnStartUp property is one of the VS settings saved in the C:\Users\username\AppData\Local\Microsoft\VisualStudio\17.0_XXXXXXXX\Setting\CurrentSettings.vssettings file.
You can find this property by opening the CurrentSettings.vssettings file with Notepad and search for OnStartUp key word. The values are usually 1(Start window)/2(Most recent solution)/3(Empty environment).
Changing its value may be easy, but VS may not refresh and import the settings from the .vssettings file immediately.
Besides, this setting can be exported(or imported) almost separately from Tools > Import and Export Settings… > Export selected environment settings > All Settings > Options > Environment > #13216(for VS 2019, it should be Startup).
Best Regards,
Tianyu
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