Hi Bh,
You may need to contact us at the following link to be able to get the help you need to get logged into your site. Please use the following link to our Contact US form and ask for help getting you logged in:
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For now take a look at the following link to some previous forum questions with this problem:
https://www.datasprings.com/dnn-modules/dynamic-registration/a-critical-error-has-occurred--object-reference-not-set-to-an-instance-of-an-object-/
What I have seen is that sometimes a change in the web.config is needed. You may need to compare your web.config to the web.config in a fresh install of the DNN version that you have.
You don't mention what is the latest version of DNN that you are on. The latest is DNN 6. but In DNN 5.6.3 the web config needs the new setting for enableEventValidation is "True", in pervious versions it was "False".
When you get to a login point you may look at the following:
If you look in the Event Log, you should see errors. If you look, you might get an idea as to which module is generating the error. Knowing that would be quite helpful. If you want to copy/paste an error message here, I'll take a look at it.