Maybe more clues will emerge from this next test, although I have not so far rebooted.
- Made a trivial test file by adding these lines to the Blink example. The Uno is on port COM5.
Serial.begin(115200);
delay(200);
Serial.println("TestSketch");
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Plugged in another Uno holding another sketch.
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Opened the serial monitor. Message 'Board at COM5 not available'.
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With Tools > Port chose COM9. (Question: does port suggested always depend on the sketch and board, not the physical port actually in use?)
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Opened COM9. Nothing printed. No Tx light flashing.
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Closed TestSketch. Unplugged the 'unknown' Uno.
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Renamed TestSketch as TestSketch2. Identical otherwise, and I will use this as the 'unknown', confident that it should print its name if the procedure works
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Loaded Blink. Closed TestSketch2, so only Blink now loaded.
9.. Plugged in TestSketch2 again. Shown as COM9.
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Opened monitor, which correctly showed its name.
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Unplugged it and plugged in the previous reluctant sketch. Tools > Port listed COM5 and COM9, with the latter marked as the correct one.
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But as you see from the screenshot COM9 is now unavailable.
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So not surprisingly the opened monitor now does not report anything.
I'm wondering whether the arbitrary 'base' sketch must always be associated with the same port as the unknown 'tested' sketch? If so, a Catch 22 sitiuation?