If I touch on “search” on my tablet then a window is opening. But as soon as I touch into the window to activate keyboard, the window disappears again. No chance to search. Has anybody an idea? Thank you 
I have the same problem. The solution I found is to type what I want to search somewhere else, then copy it and paste it on the search field. Not very practical but it works. 
Can you tap/hold the search link then click open in a new tab/window with your browser?
That’s how I mostly use it on Android, or use the main forum search link at the head of the page.