In my case it was a bad download of the updater (the download got interrupted). After I deleted the contents of the ...\MTGA\MTGALauncher\Updates folder, the game launched successfully.
My guess is that it probably won't work for everyone, since the error message is so vague that I expect it to be the same for any error that could occur. Honestly, the only way to troubleshoot is to look at the log and figuring out what's going on from there.
CuddlySpider#07221
In my case it was a bad download of the updater (the download got interrupted). After I deleted the contents of the ...\MTGA\MTGALauncher\Updates folder, the game launched successfully.
My guess is that it probably won't work for everyone, since the error message is so vague that I expect it to be the same for any error that could occur. Honestly, the only way to troubleshoot is to look at the log and figuring out what's going on from there.