After the workflow setting node Email_trigger_IMAP runs normally for a few days, an error as shown in the figure often occurs. Close the active and reopen it, and it will return to normal
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What version of n8n are you running at the moment and how is deployed? There are some issues with the IMAP node at the moment like if the connection to the server is dropped it can crash the instance although a fix for this has been recently merged so should help.
n8n should try to activate the workflow again so I am not sure why it is isn’t doing it but maybe the logs will have more details.