savedir should be './data', that and the full permission for IUSR/ IIS should do it.
As far as i know everything in DW is relative.
I have now six instances of DW running on different windows systems , and i have/had no trouble at all.(*)
But when you have to use a path( because you want your data and config out of the default server path) it should be like this c:/inetpub/wwwroot/dokuwiki/data. (Yes, also on windows / and not\)
Maybe a fresh install can help ?
(* Well, sometime i have trouble with FTP copying , this is mainly because of the many very small files)
Here is the help page :
https://www.dokuwiki.org/install:iis