I don't know how and why, but the issue is gone as silently as is came.
Because I did not change anything at the settings and configuration of the system and the SMB tranfer speeds are back at~105MB/sec for up- and download.
Unfortunaltely I can not state in detail since which OMV update SMB speeds are back again at ~105MB/sec. for upload and download, since I used FTP to transfer larger amount of data after SMB became slow for reading.
Currently OMV 4.1.32-1 is installed.