Hi Tom & Frieda. Welcome to the Uni. I assume you are working alone with Frieda. Clearly she believes you're asking for backing up so let's see if we can devise a simple & safe way to show her sideways is good too. So, forget the mounting block for now. You need a corner, some old tyres & a lightweight pole. Pile the tyres up next to the fence, wall, hedge of your corner leaving enough space for Frieda to stand calmly ( not too wide or she'll want to turn but not too cramped ). Place the end of your pole on the tyre pile, which needs to be high enough that the pole is at the height of her barrel not her legs. Make a funnel shape with the pole. If Frieda is used to 'clicking' for forwards movement choose something else - the sideways instruction needs to be clear. Back her in, raise the pole, give the command & gently & slowly use the pole to guide but not force her. Be generous with your praise for any sideways movement. On the first day, quit once you get 2 steps to the side. By day 3 or 4 you may be able to leave out the pole & work only with the command. Hope this helps. Cheers, Jo.

Hey Jo, at first thank you for your help. 
I´m not completely sure that i understand what we should built and do.
Am i right that the trick is, that Frieda gets driven by the pole to the side? Should she wear to reins and the halter or only one rein? 
Perhaps you could describe me the excact place of the wheels this is for me not 100% clear. Then I will try.  
What i did until now is a normal backup straight and in a with poles builted corner. There the turning is not a problem. On normal groundwork she did the turning of the hindquaters in both sides with no problem? 

I believe she doesn´t understand what she should do. Frieda is a dressage horse and not used to the sidesteps and backup so much...

Hi Tom. Sorry, I thought I had been reasonably clear but I will try & be better.
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Corner of an arena, school, field - approximately 90 degrees. In the corner, about 7 feet from the long wall/fence build a pile of tyres so a lightweight pole, one end balanced on top, is about the height of Frieda's ribs when you pick up the other end. Depending on how tall Frieda is the gap might need to be a little bigger ( so she can't easily turn round at the narrow end ). Lay the pole so the gap is greater at the open end, about 12 feet. Use a halter & single long line. Back her in, ask her to stand, lift your end of the pole. Pause - give her time to understand she has a wall/fence on one side & one behind her. Use your chosen command for her to move sideways, different from the go forwards ( or she'll be confused as to what to do ). Gently press her ribs with the pole. Keep the pole in place while she processes the situation. Frieda will think ' he wants something, he's giving a command that I don't fully understand & there's a pole along my ribs '. She will try forwards so you calmly stop her, she will try backing but you calmly stop her. Each time repeat the command & gently press her with the pole. Once she realises forwards & back is not getting her out of pressure she will try sideways. Be very calm & patient - Frieda will work it out. As soon as she moves even a step to the side be VERY generous with praise. 
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To clarify, you are NOT using the pole to force Frieda. You are setting up a situation where Frieda is encouraged to explore other options. You may need to set this up with the pole along her other side - she may think she goes sideways in only one direction ( always left or always right ) otherwise. Once Frieda knows your command means sideways & no longer needs the pole at all you can move to the mounting block. 
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The pole balanced on tyres means if the horse panics the pole simply rolls off. If working in a field avoid using an area with barbed wire or electric fencing. This needs to be a totally pain free exercise, even accidentally. By reducing the horses options of response THEY choose the right behaviour ( give the response we want ). Hope this helps & keep in touch. Cheers, Jo.