I have looked at the log. It begins on 24/Jan.
Mike-> Probably because on 20 January, Google replaced my 2012 Nexus 7 due to Android 5.0.2 totally trashing it. All apps were reinstalled around that time.
I am providing the dates and times in case that helps jog your memory about anything that changed either on the calibre machine or your home network.
On 24/Jan you were using automatic connections (no IP address in CC), and several connections worked.
Mike->Yes, I left it 'as installed' until I ran into trouble. The reason I check the IP address and port when this happens is that the CC error msg suggests that as a possible cause. But, matching addresses with Calibre is not enough -when it fails- to correct the issue. Sometimes (rarely) restarting Calibre can do it, other times a laptop reboot is required.
The first failure I see occurred on 26/Jan at 12:37. Calibre is clearly running because it responded to the zeroconf (MDNS) query from CC, providing an IP address and port. However, the wireless device driver in calibre did not respond to CC's broadcast query, which means that either calibre didn't hear the broadcast or that something prevented it from responding. At that point you entered into CC the IP address of the computer running calibre. It still failed, and continued to fail until you did something on the calibre machine at 12:41 that took around 20 minutes .
Mike-> Not sure why it took 20 minutes, but I strongly suspect I was doing a reboot. Probably shut down email, and skype, which I use almost continuously.
At this point connections worked until 28/Jan at 17:12. You did something between 17:15 and 17:26 that fixed it. It worked for 2 connections then failed again at 17:37. It worked again at 17:46.
Mike->Reboots.
The next failure is on 30/Jan at 10:08. There were 2 failures, then it worked at 10:10 and continued to work until the end of the log.
Note that you didn't restart CC between most of the failures, which implies that the problem is on the calibre side.
First, I suggest that you remove the IP address from CC (reset to defaults). The CC log shows that if an auto-connect fails then a connect to a specific IP address will fail. Having CC send the broadcasts could help debug what is going on.
Mike-> Done, just now, and tested. It connects.
Guessing (and only guessing) I suspect that some network security suite on your computer
Mike-> Windows firewall, and Avast Antivirus. Avast is not logging any warnings or suspect events. is deciding that calibre is doing unusual network things and is therefore evil. Some of the security software watches for connection or traffic patterns. Have you recently updated or changed this software?
Mike-> No, although windows update is running, and Avast updates its signatures and occassionally the engine as well, of course.
Another possibility is that the computer is running out of network resources. Is it possible that some kind of download manager might be running when the failures occur? Or some kind of torrent manager?
Mike-> No, and don't have or use any. And typically when this happens, I sit at the PC and try a couple of times, including restarting Calibre, and then give up and reboot. When I am doing that, I am sitting less than a meter from the wifi access point.
A calibre debug log might provide some more information. In particular it would tell us if it received the broadcasts but failed to answer or if it didn't receive the broadcasts. Instructions for getting a calibre debug log are in our FAQ answer
"How do I get help?". Because the failures are intermittent you will need to start calibre in debug mode and let it run that way until a connection fails. At that point send the calibre log to me at cc_debug_logs at charles dot haleys dot org, and also send another CC debug log.