Ok, problem solved. Connection to iSCSI Targets works instantaneously now, reboot and re-connection to iSCSI Targets works flawlessly as well.
What was the problem? Well, it all depends on how the Microsoft iSCSI initiator gets configured ...
1. Lets assume, everything on the HPE MSA2040 side is completely configured and the corresponding Host Interface has an IP-address
2. First we configured the iSCSI Target IP by clicking 'Discover Portal' under the tab 'Discovery' using the default Port 3260
3. In the 'Discover Target Portal' dialog, we clicked the 'Advanced' button
4. Then we selected 'Microsoft iSCSI Initiator' as 'Local adapter' and explicitly selected the iSCSI Target IP Address for the 'Initiator IP'
5. The HPE MSA2040 iSCSI Target IQN is instantly visible under the tab 'Targets' in the Microsoft iSCSI Initiator configuration with the Status 'Inactive'
6. Selecting the IQN and clicking 'Connect' in the 'Target' tab of the Microsoft iSCSI Initiator configuration opens the dialog 'Connect to Target'
7. In the 'Connect to Target' dialog the IQN is displayed, here you can decide to reconnect the iSCSI Target after reboot and to 'Enable multi-path'
8. And there is an 'Advanced' button again, not choosing 'Advanced' and clicking OK results in 10 minutes connection time for the iSCSI Target
9. Choosing the 'Advanced' tab, you have the additional option to select the corresponding 'Local adapter', the 'IP Initiator' and the 'Target Portal IP'
And point (9) really makes the difference. Selecting 'Microsoft iSCSI Initiator' as 'Local adapter', selecting the IP-address of your local iSCSI Ethernet Adapter for 'IP Initiator' and selecting the IP-address of the HPE MSA2040 Host Interface, where the local VBR Backup-Server is connected to as 'Target Portal IP' results in an immediate connection to your iSCSI Targets of the HPE MSA2040 without any noticable delay.
SAN-transport speed with a physical VBR Backup-Server as the Proxy (4 Cores and 64GB of RAM) on a direct 10GBit/s iSCSI connection to the HPE MSA2040 storage, results in 420Mb/s transfer speed using 2 concurrent tasks. Using 1 concurrent task results in around 250-300MB/s transfer speed at max.
Not bad for a 3 year old storage system fully equipped with 600GB SAS-drives with 2 RAID-10 and 2 RAID-50 Vdisks.
Though we must say, the two virtual HOTADD-proxies in our ESXi-hosts we used before, weren't bad as well. They pumped between 150-400MB/s of data (using deduplication) through their 1GBit/s vNICs. They will mostly be used for restores in future, as all our HPE MSA2040 LUNs are read-only presented to the physical VBR Backup-Server for safety reason.
Hope this might be helpful for some guys.
Using Veeam Backup & Replication 9.5 Update 2 on every backup server here!