The realpath: No such file … error also occurs on regular launches; started to occur somewhere after a WPE version bump. Didn’t have the time to investigate yet, but it does not seem to cause any error.
Which version of the snap are you currently using (architecture + revision)? Also, there’s a bug with mount namespaces which is fixed as of 2.42.3, but was initially behind a feature flag. Quickly scanning release notes, I don’t know whether 2.43 enabled the fix by default.
And what @alan_g says 