Passing a memory test doesn't mean the RAM is good. It's very possible that the conditions to trigger the failure aren't being reproduced by the loading in the test. The fact that there are checksum errors means that SOMEWHERE in the path that data takes there is an issue. Easiest to test is RAM modules, because it's easy to remove a portion, test for the errors to reoccur, switch to the unused modules, repeat. If the errors still occur with either half, the next likely candidate would probably be motherboard, or possibly even the memory on the drive. CPU is also involved, but that usually effects more things.
Until the checksum errors don't come back, you have a memory issue. Doesn't mean the RAM is bad, but that's the first logical place to troubleshoot.
Also, if you are unhappy with the peer to peer support, Unraid does have priority paid support available. https://unraid.net/services