causes problems on removal

Since WPtouch operates 100% on your server, there are no elements left to display a mobile theme once it has been deleted. You may want to check if your web host (or other service provider) maintains a page cache, also.

what is wrong with this plugin?

If you were running WPtouch and you had caching plugins installed, they might have cached pages with WPtouch showing that were still showing when you deactivated WPtouch. The best way to stop that is to clear your cache plugin’s caches.

If you want to use them with WPtouch, we have setup guides that can help you easily get them working together nicely, available on our free support site:

https://support.wptouch.com/support/solutions/articles/5000537668-configuring-cache-plugins-for-wptouch