It's very likely your addon is still the cause, unless the code you provided is all there is. The tainting addon will be correctly listed as the offender in most cases, and I'm guessing the stack trace that you didn't include will say this error originated from UIParent.lua.
There are a couple of different things that can cause this:
1. The terrible, but unfortunately common, practice to use no-op functions to replace functions on widgets to nullify some change they don't want in their UI. E.g.
Lua Code:
PlayerFrame.SetPoint = function() end
Don't do this, and if you have done this,
use this approach instead.
2. Calling Lua-implemented functions that bears taint with it to a table or local values in the default UI. E.g.
Lua Code:
AutoQuestPopupTracker_AddPopUp(questID, popUpType) -- implemented in Lua, not safe to call
AddAutoQuestPopUp(questID, popUpType) -- unprotected API function, safe to call
3. A combination of the above, where you replace an API function to gate or nullify the changes it attempts to make. If the function is called from a code scope that eventually does something secure, the taint spread is revealed.
If you're not guilty of this, your addon is not the cause. Like I already said, the code you posted has nothing to do with the error you're getting. Just because it happens to be Boss1TargetFrame doesn't mean it has anything to do with you moving the regular TargetFrame around. They're completely unrelated.