Hi there, I'm working on the same project. And since this forum post, the error did not occur again for about a month and has recently occurred 3 times in the last week.
The instance is a fusionauth instance.
(auth.*.ch is a CNAME (alias) for bnjmvfriojf0pzpzhtmmz6xf2sgl6b.durable.fusionauth.io)

The access where the error occurs was via browser (firefox to be exact if sentry is to be believed).

There are no recent changes to the CORS config.

For most of our users this does not seem to be an issue. It happens very isolated for a handful of users.