Can't open .aspx files in Designer View

[greglazor]

Hi,

I've recently been hired to make ehancements to a web application. It was originally maintained by consultants in India. Any correspondence with them takes forever as they are 12 hours ahead of us (I'm in Florida) and it's currently night there (and when they're at work, it's night here). So hopefully I can get an answer from you guys.
Whenever I try to open an .aspx file, I get the following error: "Session state can only be used when enableSessionState is set to true either in a configuration file or in the Page directive." And I only get the HTML code and not the Design tab.
The last correspondence we got from the Indian guys said that this was because the UI files inherit from a custom base class which prevents the .aspx files from opening in the Designer view. They said "you can remove the inheriting for the timing when designing the page, when you're done, reinherit the class." 
That's about as specific as they got. I've done all I can think of. I've added 'EnableSessionState="true"' to the Page directive in the .aspx file. I've tried to disinherit from the base class. I've uncommented and commented code that appears to be used for timing. I don't know what else to do.
I doubt I'll be able to get a specific answer without actully providing the code, but can anyone think of anything in general that might cause this and a solution for it?

Any help will be apprecited.

I didn't find the right solution from the Internet. 

References:
https://arstechnica.com/civis/viewtopic....0&t=529602
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Thank you.