The additive tank you are talking about is for eolys fluid, also known as PAT fluid by other makers, notably Peugeot-Citroen whose fluid is a lot cheaper by the way, and was unique, in terms of the Touareg, to the emissions control system on certain V10s.
From previous posts on here over the years, I suspect most V10 owners with an eolys tank don't even know it exists until, after around 100,000 miles [some say 100,000 kms] a previously unseen low level warning light pops on, or, if - like you - they have a VW dealer/mechanic who actually knows about these things [and, trust me, some don't!] advises the owner that the 5 year life of the eolys fluid is up and wants to refill the little tank at a humungous - and totally unexpected - cost!
It is NOT, repeat NOT, ad-blu as some mechanics and others may attempt to tell you!
As above, this fluid is for the exhaust and nothing whatsoever to do with any electrical problems you may have.
Re-reading your post, I don't know what the eolys warning light looks like so get your handbook out and research it.
EDIT: Found this thread and the first post seems to describe your warning light and engine fault:
Does look like all you need is some fresh fluid. If so do make sure they drain the old stuff first before putting in the new.
And do a bit of research on cost too! From memory I think someone in Australia confirmed you can use the cheaper Peugeot stuff or possibly bought the VW stuff from a much cheaper source.