I was having the same problem as everybody here for over a year, i thought I had tried everything. Reading this thread, though, I thought that MAYBE THE CULPRIT WAS THE NEW EXTENSION ITSELF (it updates in the background, leaving us unaware of the change: thus the « it broke suddenly »).
So I connected an old MacBook that had an identical system, but had never been updated to the new Web Clipper extension, and lo and behold, after installing the old extension (6.9.0-b.25 (5134137/1.1.0.926). IT WORKS!
This was driving me crazy and I feel so much better now. Thank you all for putting me on the right track, finally.
—mh
MacBook Pro running TIGER: running High Sierra on the same machine, with the new clipper, never caused a problem.