@maartje just like I guessed, there was sandboxing involved, I found it here:
"C:\Users\<me>\AppData\Local\VirtualStore\Program Files (x86)\Mastodon VB6"
(I chose the folder to be called "Mastodon VB6")
If I run it as admin then I get the ini file in its usual place.

Still can't solve the error though, I tried a bunch of Windows compatibility settings.

BTW I used this to easily generate an access token: https://takahashim.github.io/mastodon-access-token/