I just got my macmini working again, rebuilt it, added macfuse, go, upspin sources and rebuilt. Upspinfs works for me:
#!/bin/bash
...
total 40
drwx------ 1 presotto staff 0 Dec 31 1969 Group
-rwx------ 1 presotto staff 18944 Dec 14 2017 apps.tar
drwx------ 1 presotto staff 0 Jan 30 2018 bin
drwx------ 1 presotto staff 0 Nov 13 2017 flying
drwx------ 1 presotto staff 0 Jan 30 2018 pkg
drwx------ 1 presotto staff 0 Dec 31 1969 private
drwx------ 1 presotto staff 0 Dec 11 2020 public
-rwx------ 1 presotto staff 910 Jul 23 14:55 quux
drwx------ 1 presotto staff 0 Dec 31 1969 src
drwx------ 1 presotto staff 454 Apr 25 2018 testing
First attempt failed because I still had an x86 cacheserver.
My recommendation would be to turn off the cacheserver (if you are running it) AND remove all files in your cache (both the cacheserver and upspinfs have all sorts of cached crap).
If that works, try it again with the cacheserver back on.