Upgrading 15.4.13 crashed - could not use anymore

I had to downgrade back to 15.2.11 because after the upgrade to 15.4.13 PRTG kept displaying connection error. I have the latest full release on my monitoring server which is working fine.

After downgrade all is fine.

This issue affected by my iPad and iPhone which are running the latest iOS release. 9.2

CebuReview about PRTG - Monitoring