In general case I didn't change any code in web.config except the machine key and SQL connectionstring when I upgrade DNN.
I didn't modify any code when I install a module, too.
This issue happened on these two case.
The code I wrote here is what I do to reslove this error when I met it.
The twice here I mean.When I got this error, I
Change
<customErrors mode="RemoteOnly" />
to
<customErrors mode="Off" />
and reload the page.
This error gone
After, I change back to
<customErrors mode="RemoteOnly" />
So, the web.config should be the same as the original one. But I got error when I used the original web.config and without problem when I used the final web.config. But the original one and final one are identical.