There is actual an issue with SMBv2 and read chunk size, see SMB connection issue after June update - #40 by JimKnopf.
A fix of the issue in this article will be available, soon (or you can test it like mentioned in the article using the development staging).
If you could set your OMV NAS to use SMB3 as maximum version and set the mediacenter accordingly, this would be the best solution also regarding performance.
Another temporary trick till this fix will appear for all, could be to reduce the mediacenter max. SMB version to SMB1 for a short while.