Upgrading the Readynas Pro 4 (RNDP4000/4220) with 4 x 8TB disks

So after an initial successful upgrade from OS 4 to 6.1 and 4 x 8TB drives we've begun to experience some access issues with our Readynas (RN). The error is repeatable as per the steps outlined below (although we're not exactly sure which steps/combination of steps restores access) and the only workaround that will alleviate our issue is to login to FRONTVIEW, select the SHARES panel, EXPLORE a few of the shares and then click on REFRESH. This has the effect of 'unblocking' whatever is causing the RN to lock up but it then functions normally thereafter.

Error steps

1.    Boot the RN
2.    Access the RN via file synchronising program (Goodsync)
3.    Files begin copying (70/80 MB/s) for 5 seconds before slowing down to a halt.
4.    RN led activity light is still showing activity but eventually (after 10/20 seconds) the unit freezes and becomes unresponsive (LED won't light up either) and only holding down the power button to force a shutdown enables us to reboot the RN.

We still have to use the 'workaround' before doing any kind of accessing of data on the RN otherwise if we were to initiate Goodsync first (like we did successfully without any issues in OS 4) it will cause the unit to lock up. Then we're back at step 4.

Current Frontview Settings:

5.    Model: Readynas Pro 4
6.    Properties > No Quota Size has been set
7.    Network Access > Only SMB access has been activated
8.    Snapshots > are turned OFF
9.    File Access > Share access has been restricted to admin and one other user and both these users are checked for R/W access under all relevant share listings (i.e. the default EVERYONE and USERS remain unchecked).
10.    Grant Rename and Delete Privileges > Checked
11.    No spin down settings have been activated
12.    Firmware: 6.6.1
13.    Antivirus: Disabled
14.    Available free space: 11.73 TB

Any ideas what could be causing our issue please?