The problem was that I did not have xotcl-core installed.
I hit the problem on a website that had been upgraded to 5.10.0 from 5.9.1, and didn't have xotcl-core installed.
Then when verifying the problem, I did a fresh install of 5.10.0 and attempted upgrading it to 5.10.1. But again, I failed to install xotcl-core on the 5.10.0 fresh install.
If I install the xotcl-core package on the 5.10.0 version, then the upgrade to 5.10.1 runs without errors.