I am also having a similar problem with both downloading existing data and studies not completing, details below:
19/04 we tried running participants in 3 different studies - the first participant went fine but the subsequent participant in that same study (+ two attempts for two different studies) returned a 504 error after the participant finished the study. Resources seemed to load fine for all participants and it seems the error is related to pushing the data to the server (as all 4 credits were consumed but 3 of those are considered as incomplete and/or aborted despite all participants finishing the study)
20/04 there were no issues with running (different studies but same account) but was given a different 504 when trying to download the database from one of them afterwards (Pavlovia still seems to be trying to fetch these data after 2+ hours). Now when trying to run the studies from 19/04 (later today) we are again receiving the 504 errors
Both errors come from nginx so it seems to be a server issue
See also: https://discourse.psychopy.org/t/error-when-uploading-participants-results-but-data-are-saved/34437https://discourse.psychopy.org/t/pavlovia-started-not-working-today/34435/2