Hi Jeff;
Yes, I have seen that error before and many others. Looks like you are trying to use the WebForms feature from the overall description of the issue.
1) Webforms is EOL and was removed from the product in PB 12.5.2
2) Webforms does not work well with W7
3) Webforms will not work with W8
4) Webforms will not work with IE 11
5) Webforms has many issues with IE 10
I am not sure what to tell you on your computed column issue. I highly doubt that SAP technical support will assist you in resolving this as the feature is no longer supported. Also, Sybase removed all the older PB 11.x MR & EBF's from their website when they EOL'ed Webforms. So I am not even sure if you can get the 11.5.1 MR to see if that helps you.
If you are open to another route, I would strongly recommend having a look at the Appeon Web product and porting your Webform application over to that technology. Appeon already supports 64 bit, IE 11.1 (W8), mutithreaded / reentrant code execution, etc. FYI: http://www.appeon.com
Regards ... Chris
PS: I switched to Appeon in 2008 and never looked back!