We too are experiencing the same issue. As per xnau’s advice, we install a php log error widget, and the results are seemingly that the tables just don’t exist, and aren’t getting created:
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WordPress database error Table ‘wordpress-mu-public.wp_19_participants_database_fields’ doesn’t exist for query
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However, we had an admin user inadvertently crash our multi-site server instance with a back-up plugin, and I don’t believe of our plugins currently have the capability to access the DB for some reason, we are still working on this issue. So this may or may not be related to the other users.
