Installtion is not possible since the controls of the set-up.exe dialog are not reachable

I have spoken to the RoboHelp team and this is outside their direct control. It is a standard dialog used by all Adobe products. They will raise the issue with the team responsible for the dialog.

Meantime, tab twice and press Enter. 🙂

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My site www.grainge.org includes many free Authoring and RoboHelp resources that may be of help.

 It's possible the screen size has been coded for English and doesn't take into account the longer nature of German words, and I would consider that a bug. 🙂

You can request features and report issues here: https://tracker.adobe.com/
(You can also search in the issue tracker to see if it's been reported before, and upvote the previous report.)
Post the item number in the discussion so people can easily vote if they want the same functionality.

Also, if it happens again, or someone else encounters it, check the Windows display settings and make sure the Scale and Layout text size setting is set to 100% (not 150% or 200%, etc). I can't say this is the cause, but it can cause weird display issues in programs that aren't programmed appropriately.

Haven't seen that before - have you tried just hitting enter to get it started (& you're running it as an Admin, right?). The field that's cut off is the standard c:\program files\ install location IIRC.