Is it possible to repair this myself or do I have to send it to festool? It looks as if thé purple back should be taken off in order to replace it, but I dont know if thats easy to do?
If it is possible to do this repair depends entirely on yourself. I would do this repair myself, but I have been doing this my entire life, with a fair share of screw-ups during the learning process.
I think it is very easy to repair power tools, especially when you're not working on the electrical components. Most of the time with Festool you only need a screwdriver, T15 or T25, and you can disassemble the entire tool. Make sure to take detailed pictures so you know how to put it all back.
Festool makes it also easy for you to find spare parts, and gives you excellent exploded view drawings in their spare parts catalogue. See the menu of this website "Additional Websites > Spare parts catalogue" to find your Kapex.
So if you're a bit mechanically inclined it should be no problem to fix this and save yourself some bucks. In the worst case you can still send it in to Festool.