Could you share the filter log when the email started processing, there we can figure the cause.
Also SQL logs in separate file.
Did you check RBE forms? especially RBE:Message and check the activity log.
I have faced the same error but with a different scenario, when email send from Remedy and I have checked the email which I put in configuration that I send to it, the email is deleted automatically by Remedy from outlook inbox, as I have disabled the mailbox configuration (outgoing & inbound) and try to send email that email the email is not deleted and didn't get that error message in inbox. I have enabled the filter log and found that error in logs but in different filters and form. I believe in my case it's with "NTE: SYS-NT Process Control" form
foremail2.log 46.8 MB