@Faber, as mentioned by Stig, CROSSBOLT, mouse, a HELICOIL insert would be the best and most durable solution.
2nd best would be drill through with 5 mm, retap 6mm, put a longer 6mm bolt
3rd best a bigger (8mm) thread
4th best (if all other fails) driling completely through and put a long 5-6 mm bolt and nut there.