[Koha] 500 error when choosing patron for purchase suggestion

[Jesse Stallings]

Running 21.05.02 on Debian stretch 9.13. When selecting "set to patron" or
"select manager" while entering a purchase suggestion from the staff
console, a 500 error rather than a patron search box appears in the popup.

This issue seems related to bug 26738
<https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=26738>, where
suggestion managers and patrons were unable to be saved, but I'm unable to
get even that far. I'm not sure how to troubleshoot this—has anyone else
run into this issue?

Created bug report 28946
<https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=28946> last
week, but hasn't received any attention.

Thanks!