I confirm the problem.
Same LS version
MySQL 5.5
When selecting "Partecipants" I'm not presented the option to choose between "Switchto closeda-ccess mode" and "Continue in open-access mode".
I attach a screenshot.
The problem doesn't occurr in 6.15.3+250708

immagine.png (31,834 bytes)   

immagine.png (31,834 bytes)   

I assume, that something is wrong with the binary value for open vs. closed access mode. Error can be easily reproduced creating a brand new survey, creating a single question, creating a dummy participant table (e.g. with 10 respondents), activate the survey, and the survey is open access, although there is a participation table. Maybe this error is associated with a new feature in the cloud version of LS.