I have experienced the same problem after updating to 4.20.
But I think the problem is already solved for the next release, according to this bug report:
https://bugs.launchpad.net/calibre/+bug/1886231
P. S.: I've tested it running from source, but it didn't work. But, IIRC, the Viewer is built on the fly in that case, so it might not work 100% as expected.