I just stumbled in the same problem.
I had to follow these step to actually fix this and it seems none can be
skipped:
1 remove offending sources from /etc/apt/source.list
2 zap /var/lib/apt/lists
3 disable the proxy
4 apt update
5 add back the offending sources from /etc/apt/source.list
6 apt update
7 enable the proxy
I had to put together all the receipt I found on the net to succeed.
Removing the offending source, apt update, reenabling the offending
source didn't work. (1 4 5 6)
disabling the proxy alone didn't help 3.
2 3 4 5 didn't work either
I had the chance to fix this on 2 machines and on the second one I tried
to anticipate to re-enable the proxy just after putting back the sources
(6) and it didn't work.
I hope it helps to find the problem.
--
Ivan Sergio Borgonovo
https://www.webthatworks.it https://www.borgonovo.net
