Ok, this came up the other day here. And off and on for many years, I seen this issue.
Now, I have trouble re-creating the issue!
I suggest this:
close all pages and code behind pages
do a clean project.
Now, delete all files in the bin folder
(ONLY do this assuming this is a asp.net web site application - one with .sln file).
So, right click on the solution explorer, and choose open in file explore.
eg:
browse to the bin folder and open it. Now, delete EVERYTHING inside. (so, ctrl-a, and then hit del key).
Now, do a rebuild solution. This:
Now, try opening the aspx page with the ascx control. It should work.