Rules error

We have Outlook, not the online version (our version is 2016 (16.0.5071.1000) MSO (16.0.5056.1000) 32 bit), and we have set up simple rules where every email received for the specific mailbox (client-only rules) is replied to with a specific template so the rule looks like:

"Apply this rule after the message arrives

reply using C:\..........(location of .oft template)"

This is our work-around for when staff are on leave and we need an out of office reply - we do not have an exchange email server - our emails are all POP3 accounts hence we have to open Outlook for that particular staff member so the rule emails get sent.

The rules seem to work fine until the computer remains idle for quite some time, possibly over 2 hours, and then the rule then is highlighted in red and has (error) after the name of the rule.  For any emails that have come in during the time it states their is an error with the rule, if there is an attachment, it seems the attachment can not be opened and saying it is corrupt (the attachments are mainly PDF format).

We have tried deleting all the rules and setting them back up, done a repair on the .pst file, have made the PST file old and created a new one but nothing seems to work.

Is anyone able to assist with this as we do need these rules to work even when the computer remains idle and we are not looking at changing over to a hosted exchange server at this stage.

Thanks.