As of dbFront 18.104.22.16892, dbFront now correctly handles renamed/dropped fields that appear in lookups.
Previously it was determined that renaming a primary key or other column on a lookup table could cause dbFront to get confused and reject the relationship going forward due to the presence of now old invalid configuration.
This issues is now fixed.
If you ever encounter issues with relationships it is good to open the Log Viewer and search for the word "Relationship". dbFront should log any details there.