Comment 1 for bug 1829159

I have the same problem on a machine where I installed Ubuntu 19.04. On an older machine, which is updated to Ubuntu 19.04 this problem does not occur. So I believe on the older machine the file was copied correctly when initially installing octave.

Execute octave once with sudo seems to fix the problem for some people:
https://forums.fedoraforum.org/showthread.php?321255-Gnu-Octave-quot-Could-not-copy-help-collection-to-temporary-file-quot
I have not yet tested this though, I am hoping the bug is resolved with an octave update.