Varban, I assume the ACE Scheduler runs on the local machine as well. You need to make sure the local machine can write to the PI point, e.g. a trust is configured for your local machine on the PI Server.
Thank you for your reply, Andreas!
Yes, ACE Scheduler is running on the local machine. I added new trust in section Mappings & Trusts of SMT with IP address, network path and net mask of the local Machine and... I was just about to write that this doesn't help ... but after restarting the computer it works!!