Error while trying to synchronize Audio and MIDI

OK--help not being forthcoming, I experimented, and the problem seems to be solved. Here are the keys:

1) Increase the I/O buffer size to at least 256 samples. I'd made it 128 to insure low latency; but that was too low. Latency is not a problem at 256 or even 512, and evidently these MIDI plugins need the larger buffer.

2) Under "MIDI preferences," click the MIDI Sync tab, and then click the "MIDI Sync Project Settings..." button. In the MIDI Clock section, tick the "Auto-compensate Plug-in Latency" box. I'm not sure what that contributes to the "Error while trying to synchronize Audio and MIDI" error, but it looks like it should be key. However, unless the buffer is large enough, ticking this "Auto-compensate" box doesn't help. All I can say for certain is that ticking it doesn't seem to hurt anything.

Ah, Erik. Such useful advice. Indeed, I did search on this topic but, unfortunately, not one of your 20/30/40 treatises came up. I'm sure they were all brilliant, and that any one of them would have resolved my problem. Thanks again, though, for reminding me to search before I post a question.

Pancenter--Many thanks. All of this is useful to know. Although enlarging the I/O buffer worked for this iteration, who knows for the future? I'll try the settings you suggest to to try to preclude further problems.