You need to track down where the error is occuring.

My method is to put debug statements at the entry and exit of my functions. I generally just print the function name, indented using _ (underscore) for each level of function it is going into.

From there, I catch any errors that happen, display them, (prevent DNN from handling it) and then look at my output and see what function was the last one that my debug went into.

It's a little trickier with it happening close to page load - but eliminate some pieces of code, try to get the error for a reasonable amount of time, if you get it, eliminate some more... if you don't (again, a lot of time may be required to be reasonably sure the error isn't coming) add back in some of the last code removed.

I normally remove large swatches of code, and then slowly add code back, same idea. Remove code, test, uncomment small chunks, test... repeat.

There isn't enough information to figure out the problem from what we have, for me anyway. You need to debug it and find the offending line(s) of code and continue from there.

Use the OnBinding function, output the text/value of the items in the drop down list, the text/value of the item(s) being bound or selected. Basically, as much info as you can about the text/values being used in the dropdownlist and the data being bound - Then, catch any errors in the OnBind function (if that's where the error originates) and output "ERROR OCCURED" and you can check the data and see if something stands out.

if the items in the list change, you can look for a common element when the error occurs vs when it doesn't, ect