There's nothing wrong with enabling the syslog server mirror to flash. The message warning that appears is
Which is 100% correct
Everything in Unraid, especially once you get to VMs is "tainted" They aren't errors per se, but rather a flag / note to the kernel maintainers that the particular module / executable etc isn't the "stock" module that comes with the base kernel but has been modified. The maintainers will see this if you are ever submitting to them a bug report etc and then choose whether or not to help you because the coding has been modified.
In other words, it is not an error at all in the Unraid world and is expected.