I ran in to a similar issue in DF19.1 when I updated the code on the server from GIT and the copied the original files back in the folder to make sure all data was available even though it was not in GIT.
It was the Set External_Class_Name "DFSysLink" to "SysLink" in cLinkLabel.pkg that all of a sudden failed. But it still worked on the developer environment.
The thing that solved it was to uncheck the "Embed Manifest File" in the "Project Properties" then compile. This created a app.exe.manifest in addition to the already existing app.manifest and the only difference was the first row in the file that did not exist in the old version "<?xml version="1.0" encoding="UTF-8"?>"
The interresting thing is that after that I could check the checkbox again and compile and it still worked.
I have no clue why this is, but I thought I'd share if anyone else run in to this issue in the future.