I have seen this in V20 Release Candidate 1 and it is reproducable.
I also saw this error in Beta3.
Delete or rename "My SlickEdit Config" directory.
Start SlickEdit.
Quick Start Configuration Wizard appears.
Click Finish (no changes).
Select: Tools > Options > Languages > All Languages > Formatting
Change Begin/end style to Next Line
Select "Adaptive Formatting" or anything else to leave "Formatting" options screen.
Click OK
I then get the following error:
Slick-C Error
Invalid number argument
file=setupext.ex offset=49980
If you make the above change, then change to Adaptive Formatting, then change back to Formatting again, the Begin/end style will say Languages Differ (the change I made is lost). Clicking OK at this point again produces the error.