...I use AVG and it seems to be no problem.
AVG here too.
I have tried to turn off the email scanner earlier and there was no difference.
Ok.
Other than noticing your above, "...But then I switched on the Yahoo page to the Classic view, and the "negative vibes" error went away. I got one email then, but now again nothing since. ..."
seems to be backwards from what I would expect, I don't know what else to recommend.
I would have expected it to stop with the negative vibes when you switched
from the Classic view with the "Upgrade to the all-new Yahoo..."
When it is configured properly, it seems to work ok here.
You said you got "nothing since". This may sound silly, but was there more new mail there to get?