Dear @AndreasB ,
I am sorry to read that you encountered an issue on this occasion.
What happened was that I deployed a fix to the main server on the 26th of September, which regrettably altered the encoding of the answers given in response to queries to the pavlovia back-end server. As a consequence, those answers could not be properly decoded by the front-end (which explains why you could not see your dashboard) and the experiments (which explains the specific error message your participants got).
We noticed the problem rapidly and corrected the issue (see Accessing Pavlovia: SyntaxError: Unexpected token & in JSON at position - #3 by apitiot). This explains why your participants could run the experiments later on.
In other words, this did not have anything to do with running your participants simultaneously. I can assure you that running 20 participants again will not be an issue. pavlovia.org routinely handles many, many more simultaneous experiments.
With my apologies for the mishap,
Alain