Thanks for the ideas!
Yes we've tried vanilla server -- works perfectly. We installed Tin's mod which worked (after correcting all of the case errors he had -- Linux specific issue since Windows mostly treats them the same).
We saw the issue looked like it was calling for that library. Interestingly enough the directories /7DaysToDie_Data/Managed and /7DaysToDieServer_Data/Managed have different DLL files. Since this error is looking for a specific file, we tried just putting the file where it wanted to look for it, but it didn't make a difference.
Lastly, while searching around it seems that others have run into this ( for example, True Survival [JZMOD] -- https://7daystodie.com/forums/showthread.php?38584-True-Survival-JZMOD/page162 ), and it seems related to SDX. I know Jax is just building on other's work, just was hoping that someone might have run into it before
Again, thanks for the suggestions, we'll check out the strace, though we might not be able to fix the issue, at least we might be able to narrow down what is up.