Five days without a problem... I've marked this thread as resolved.

I have a new problem with Mercury.  I don't know whether to call it a crash or a hang but it has happened yesterday morning and again this morning.  I don't run it as a service and keep its window open on the desktop so can tell when this happens by the window being gone and no entry in the taskbar or icon in the notification area. Task manager shows both it and loader as still running.  I can end loaders' process but attempts to end mercury32 fail. 

There aren't any entries in Event Viewer except some associated with the attempts to kill its process.  The only change has been a Windows update (Win7) so if this problem occurs again I'm going to restore to the point prior to the upgrade. 

Does anyone have any suggestions on how to troubleshoot?

This is a shot in the dark, but the only time I see something similar is when explorer.exe crashes and restarts and Pegasus Mail disappears. However, it can be killed via Task Manager. This applies to running Pegasus Mail on Win7. We run Mercury on Windows Server 2008 and Mercury is amazingly stable, so I've not seen this. 

re: Windows Update - I always decline the previews (we use WSUS). I can't see any previews for August's updates but don't know which updates you are referring to.

The update we did was the Quality Roll-up from July (KB4338818).  The August Quality Roll-up (KB4343900) was released yesterday so that was installed this morning.  We'll see if it fixes the problem. 

One thing I noticed this morning is that the crashes occur daily at the same time (+/- 5 minutes).  That time is about 20 minutes prior to the daily maintenance restart.  I don't know if there is any relevance to this.  No other programs are running on this PC and no events occur at that time that I can identify (no AV scan or backup).

Sophos had an issue - might lead onto something useful for you:
https://community.sophos.com/kb/en-us/132512

Thank you my friend.  The patch that fixes the Sophos issue in included in the August Quality Roll-up that was installed this morning so fingers are crossed.

Until tomorrow...

YEA!  Mercury was normal this morning.  I'm not ready to state that the problem is resolved but this is a good start.

[quote user="Brian Fluet"]...  No other programs are running on this PC ...[/quote]

Hi Brian,

you wrote that no other programs are running on this machine. Why the Sophos fix can take affect?

Our Mercury is running on a Windows Server 2016 machine without any additional anti virus software. The A/V scans will be carried out with our ISP (1. stage) and at the user's machines (2. stage). This should be enough.

But as recently reported in another thread our Mercury hangs as soon as Mercury's SMTP Client is not able to deliver mails to the ISP mail server. Means, if there is e.g. only a very very slow connection to ISP mail server available, Mercury core is not working until all mails are being sent by Mercury C. If this happens, no local mails will be forwarded and already retrieved mails from the internet will also not processed.

The other case where Mercury is not "visible" is when starting it as a service. Mercury is working fine for a few days and the Mercury icon appears in the sys tray. But after a few days the icon is disappeared and I have no access to the GUI. But the task manager shows that Mercury is still running. Now we are starting the Mercury GUI manually and it works.

[quote user="Joerg"]

[quote user="Brian Fluet"]...  No other programs are running on this PC ...[/quote]

... Why the Sophos fix can take affect? ...[/quote]

Brian is not running Sophos. We run Sophos at our office and I had seen the notification from the Sophos Enterprise Console that a July Windows Update had caused issues and that a subsequent Update had fixed it. I let Brian know about it in case he saw anything that might relate to his system.

[quote user="Greenman"]Brian is not running Sophos. We run Sophos at our office and I had seen the notification from the Sophos Enterprise Console that a July Windows Update had caused issues and that a subsequent Update had fixed it. I let Brian know about it in case he saw anything that might relate to his system.[/quote]

Exactly.  The hint about the Sophos issue was timely because I hadn't yet researched issues with that update.  It led me to that there were a number of problems with that update.  I don't know which problem caused my problem but felt confident that the update was indeed the cause.  Mercury was fine again this morning so I'm feeling good about the problem being resolved.

FWIW, I run two instances of Mercury, one running C, D & I, the other running S &  E.  The problem affected both instances.  Both are started by loader which are executed by a batch file that runs on machine startup.  The windows of both instance are open at all times.