It seems that the transfer of parameters via URL (Key-Pair-Id, Policy and Signature) no longer works. However, using a cookie according to the following scheme works:
CloudFront-Key-Pair-Id=KKK; CloudFront-Policy=PPP; CloudFront-Signature=SSS
The new URL for JOSM is:
tms[15]:https://content-a.strava.com/identified/globalheat/all/blue/{zoom}/{x}/{y}.png