In fact, I had the same problem with the Yahoo! portion of the Contact Importer plugin, exactly as other users described here. However, I've managed to debug and "resolve" the problem after few debugs and tests. In fact, it is NOT a hosting problem or a plugin bug, instead it is actually a lack of information (maybe knowledge?) from the plugin developers in regards to possible conflicts in between the Yahoo verification process and OxWall rewriting rules. To be less technical, here is what anyone need to do to quickly and easily have this plugin fully working:

If you already have created your YAHOO APP, you must pay closer attention in the fields "Application URL" and "App Domain". It CANNOT have "www" in those fields (the "www" is what makes Yahoo fail when creating a token during the communication process). As an example, if your domain is "www.mysite.com" you must use "mysite.com" instead for those two fields.

If you have already created your YAHOO APP you may have to delete it and recreate it as Yahoo won't let you edit those fields.

It is simple as that! Despite of the fact that Yahoo may validate the domain with or without "www" in it, when OxWall try to obtain a token to access Yahoo data if fails because of the rewriting rules found at .htaccess file. As result, during the communication process only, a domain with "www" is identified by Yahoo as a different domain from the one without "www", resulting in validation error and no token (and, consequently, a OxWall debug error saying that no STRING result has been found)!

Back to Yahoo, another tip is how to properly validate the domain when creating Yahoo APP. I believe some people may have faced a little challenge doing it because (again) OxWall has a very complex .htaccess file, with several rewriting rules, and as result Yahoo won't be able to access the file to validate the domain without a "quick and dirty trick". The trick consists in temporarily rename the ".htaccess" file to ".htaccess.bak" in the root of the OxWall installation before clicking at the validation button on Yahoo APP. This way all rewriting and redirecting rules will be temporarily disabled and Yahoo will be able to direct read the file it asks users to create in the domain root folder. After Yahoo validates the domain you MUST rename back the ".htaccess.bak" file to ".htaccess" before trying to access/use OxWall once again or OxWall will crash.

Of course we could also edit the .htaccess file and make the proper adjustments to allow Yahoo to read the file, however it is too much risky for a common user to even touch on .htaccess file and a unnecessary amount of effort just to allow a simple verification process (and YES, you can delete the file Yahoo required after validation is complete).

After those little tricks and tweaks I've managed to have this plugin working flawlessly in the OxWall 1.2.2. IMO it is a very cool addition to OxWall and a must have feature for any social network site! =)