Upgraded form (6i to 11gR2) skips logon

I am upgrading our Forms 6i application to 11gR2 (11.1.2.2.0 with WLS 10.3.6 on XP or Windows 7 development PC). I'm stuck at our startup form already! The form just skips the logon process. The 6i forms are originally generated via Designer using Design Assist (similar to Headstart), so the form contains a lot of generated code (even though it's just a simple startup form), which makes it rather complicated.

I added an on-logon trigger to the form (to make sure that it's not skipped in some attached library somewhere), but this trigger never fires:

message('on logon');

logon('','');

The form continues to the when-create-record trigger on the first block without logging on. Of course I get "not connected to oracle" here.