Today I took an (v1) extension, that had worked perfectly on NAV2017 and tried to publish it to a NAV2018 instance, resulting in a cascade of errors:
 You have specified an unknown variable.SDefine the variable under 'Global C/AL symbols'.
I have specified an unknown variable, then SDefine the variable? Strange?
It turned out, that the offending variable was called “S¢gning” (Old danish code, with messed up character conversion), so the character ¢ after the S messes up the error description. The object compiles and work fine in NAV2018 FINSQL, so it seems that Microsoft have changed the way a V1 Extension is compiled at publish time that has a stricter character requirements. Changing into the correct Danish ø does not help.
Update: This was an interesting experience in wrong combinations of regional settings on servers vs. the quirkiness of the .TXT format. After going through the entire workflow and fixing setting I’m back in business 🙂
(Article added here for helping people when googling errors like this).