Have you recently patched the server?

A quick skim of the https://www.hmailserver.com/documentation/latest/?page=system_requirements documentation indicates that this product is only supported on SQL Server up to SQL Server 2016.

As mentioned above, you might have better...

Has the underlying storage changed?  Is your storage experiencing other problems? 
Are you backing up locally or remotely?  If to a remote location, has your network config changed? 

M'colleague is wise.

Here - have some code.

DECLARE @dt VARCHAR(20) = '20180426 112500';

SELECT @dt AS YourString,
   STUFF(STUFF(@dt, 14, 0, ':'), 12, 0,...

Make the first step of the SQLAgent job a query against sys.fn_hadr_is_primary_replica to see if the database hosted locally is the primary or the secondary, and then use...

Sounds like a file permissions problem.  Check that the SQL Server service account has the appropriate consistent permissions across all those files.

If you follow the steps to create a scheduled task to cycle the error log every day, and then set SQL Server to retain only ten logs, then that'll give...

Which antivirus product is this?

There's some general advice from MS about installing/configuring AV software to run on SQL Servers at https://support.microsoft.com/en-us/help/309422/how-to-choose-antivirus-software-to-run-on-computers-that-are-running

You might also want to check...

(1) Remove one of the )s from the last line.

(2) that last "FROM SOLICIT.gtm_doc" bit - does that belong to the SELECT MAX(D.sent_date) bit?  because you've already got...

The .mdf file that was mailed to you...  what version of SQL Server was that from?  If it was from a later version, then you won't be able to attach...