I've never had an issue with the flashmemory plugin causing this. Does removing the plugin fix your problem?
It was just an assumption, anyway I've scrapped the A10 board for several reasons and moved to PCDuino3Nano. The problem does not appear there, however it sometimes appears on my main OMV x86 server with the system installed to an SSD drive. not sure what causes the issue, but it appeared only once. I suppose it can be caused by OMV running from a slow storage drive like microSD.
Main Server: OMV 3, Cooler Master Elite 120, Asrock E350M1, 8GB DDR3, 64GB SSD for OS, 2x4TB + 2x2TB - Seagate, 1x500GB HGST, 1x500GB WD.
Additional Server: PCDuino3 Nano, OMV 3, ARM V7 Alwinner A20, 1GB DDR3, 300GB +160 GB old IDE Seagate Drives, running over wifi.
in my case it seems the issue is related to OMV 4. installing and setting up OMV 3.0.89 did not invoke the issue. so it seems like this:
1) HP N40L - OMV 4.0.6 failed to read file error appears after installing & setting up plugins, reproduced 3 times
2) HP N40L - OMV 3.0.89 - everything OK, cannot reproduce
3) Virtualbox - OMV 4.0.6 - everything OK, cannot reproduce
Now i cannot gurantee that i performed exactly the same steps in every case. In case of virtualbox i did not set up an additional partition on the system disk and did not change the default webui password.