Try to go through your progress bar textures that use a custom text (%c and a lua function) in the display tab, there's likely some "local x = ..." and later some aritmethic using x in there without error checks.
Edit: You could probably just search for "local x" in your WeakAuras.lua savedvariables file:
1. Open <Your WoW Folder>\WTF\Account\<YourAccountName>\SavedVariables\WeakAuras.lua in any editor you want
2. Search for "local x"
3. the line containing the match will probably have ["customText"] at the beginning
4. The aura name containing that is in ["..."] at the top of that block, for example:
Code:
["My Cool Aura"] = { <<< This is the aura name that's likely causing you trouble
... stuff ...
["customText"] = "... stuff with local x ...", <<< This is the line you found
... stuff ...
},
5. Link back your aura so we can take a look at it