Not resolved, but found a workaround. While on-site, I noticed their IT guy moved MAS 90 to different server. He edited the shortcut to point to new location, but didn't reinstall WK Setup... so Windows Registry was still pointing to previous path, which no longer existed. I removed previous WKsetup and installed again, but that didn't resolve issue.
While testing further, I noticed that the error only appears when printing a register from Lines tab of data entry screen (AP or AR Invoice Data Entry). Specifically, the curser must still be on row 1 when print button is selected. No error occurs when printing from row 2 or any other tab of data entry screen! They're on 4.3.0.18, so it may be a bug in that release (didn't confirm yet).
Initially, they couldn't get more than 2 users on without the error, even though they have 5 user license, Disabling Op locks via registry edit got everyone on, but didn't clear the error during AP/AR updates.