nadimpolemis
That means there is an email in the configured mailbox that cannot be processed for some reason or another. Since it can't be processed it also can't be moved. So that email will continuously throw an error every fetch unless someone logs in to the mailbox and manually removes it or if it is eventually fetched (maybe the first few failed attempts were due to network/server/etc. issues).
With this being said, someone had a similar issue and resolved it by going to this file and line and changing it from:
$fieldClean = json_decode($fieldClean[0], true);
..to:
$fieldClean = is_string($fieldClean[0]) ? json_decode($fieldClean[0], true) : $fieldClean[0];
You can apply these changes and see if it resolves your issue.
Cheers.