TBH, I have no idea. However, we don’t know yet if the problem is really a misconfigured proxy; maybe when the problem will be more clear, it will also be more clear why it suddenly fails.
Besides, could you post your system version, and Anki version — just to be sure I’m reading the appropriate proxy documentation 
?
I have the impression that you basically have no proxy configuration, so it could hardly be misconfigured. However, if it is misconfigured, according to this other thread where somebody had a similar problem, even when syncing, Anki needs to make local connections, not only outgoing ones. You may want to try to add localhost to that list of bypassed hosts. Alternatively, localhost is most likely to be a “simple hostname”, so checking that box might also do the trick.