It seems the update from sci-biology/ucsc-genome-browser-217 to sci-biology/ucsc-genome-browser-221 breaks because the installation paths changed from /opt/ to /usr/ (it is my wild guess).
Created attachment 216232 [details] build.log
Martin: Please don't add people to CC:, you're only annoying bugwranglers :) (but keep up the bug finding, that's great!)
Fixed in ucsc-genome-browser-223. Thank you for reporting.