Complete quest button not working (normal wow window no addons on)

[Ualaa]

Have not seen that.

If you don't launch from the IS button on the task bar, I cannot see how IS Boxer could interfere with the game.
Jamba shouldn't either, but you could try wow with every addon off and launch by double clicking wow.exe to eliminate every other option.

[MiRai]

What really helps on a support forum, is when people don't edit out their problem with a "nevermind" when they've fixed it
themselves. Someone may have the same, or similar, problem as you later down the line and need a fix, but they won't find it and
no one else may know where to point them.

[Koralon]

What really helps on a support forum, is when people don't edit out their problem with a "nevermind" when they've fixed it
themselves. Someone may have the same, or similar, problem as you later down the line and need a fix, but they won't find it and
no one else may know where to point them.

Well, long story short, if you are unable to click the "Complete Quest" button while turning a quest in, delete your WTF file for that character, problem solved.