Some helpful advice that I overlooked, unfortunately, I tried them all and no change, still file upload errors.
As you suggested, I re-tested using php 7.2.
I believe I tried elFinder a when the problem first surfaced, but re-tested anyways - no difference.
The suggestion I though surely would work, disabling CSRF, but also, no changes, still errors.
Unfortunately, I cannot risk moving to Tiki 21x until this issue is solved, as it does not work with php 5.6 (Tiki without a working File Galleries would be a severely wounded duck).
In any case, thanks very much for trying to help.