So you are trying to put, for example [ "danbooru + touhou ] into your addressbar on your phone?
My first gut-feeling guess would be is that your mobile browser is trying to parse that search of yours as a URL for some reason and is therefore saying "I have no idea what to do with this" and throwing that error.
Some more would be really appreciated:
- Which OS is your phone running? (iOS, Android, ...)
- Which browser are you using?
- What is the exact search string you are using?
- Where are you entering this string? (the big addressbar in your browser, a special other search bar, the searchbar on the Google homepage, ...)
BUt considering that your phone is the only thing having this problem, my guess would be that the problem is on your side.
Also, I just tried entering "danbooru + touhou" into my Android, it redirects to a Google search without anything special happening.