I am a manager for a group in which a mashup was added by another user. I made myself a manager for the contest to update some settings for the contest. However, I can no longer make myself "not a manager" for the contest, exactly like https://codeforces.net/blog/entry/101986.
I also tried leaving and re-joining the group as a participant instead of manager of the group to see if I would lose my manager status for the contest. This just left me in a state where I could not enter virtual participation, where the error message "Mashup manager mode was automatically disabled. You may now enter the contest." repeatedly appears, without any changes.
The contest ID is 555056.
You can try to press the button in Mashup Pages, which works for me.
However, I have another question: how can I change the settings to alter the scoring of the problems instead of having them scored as 0?
Unfortunately, I did not create the mashup — another group member did (I cannot access it from that page).
Perhaps you could ask the person who created the competition to temporarily remove you from the list of managers, and then set it back after you have registered.