This is also mentioned in this topic:
Relevent bit:
Finally, something that may not be related to the Class but should be monitored in the migration, the use of PROP:WHERE and / or SET(KEY,KEY) in the same thread as a browse or FileDropDowns or FileDropCombos based on te same table was causing
Error Code 78 - Illegal Number of parameters. The last thing I remember was commented on in the Clarion11 group on 2020-11-02 in the topic ‘Re: Size not equal for key’ by Bjørn Øyvind Holm, also on the 6th in the topic ‘PTSS 43119’. You can view the PTSS log for more information and testing programs. They solved using ALIAS there.