Meanwhile we solved the problem. If there is too many printig objects, there's a problem to check n' grant access to the com-object for the remote accaount. We found out it is not a printing problem, it was the problem of the application respectively the check of the access n' authorization to the com object. So we reduced the amount of printing threads per order, now it works. The application is a 32bit application running on a 64bit server. We certainly contact the other Software Provider to finally migrate the application to 64-bit.
Case closed. Thank you for your support.