506 errors can be caused by several different things, so resolving these entirely depends on if it's something that we can track down and then if it's something under our control or at least work around. 
 

We've fixed one of these recently fixed in 17.04 related to serial number history lookup, and another one related to EHF item references is fixed for the 17.05 update.  These were caused by bugs in the way that Atrex attempted to access the data for those specific functions.  506 errors of this kind are a pain to track down, but can usually be fixed permanently once we know exactly how to reproduce it.  

If you see 506 errors in the same area of the program frequently, give us a call and show us exactly what you are doing.  We can then take a look at the program code and see if we can isolate and fix the issue for you.
 

Another cause of the 506 error is long running functions or performance issues on the server side that don't let the session manager handle the lock request quick enough.  These are much harder to track down because it's not necessarily Atrex causing the problem.  We've seen Anti-Virus software scanning the Atrex database tables, and live backup software such as Carbonite, lock the database tables long enough to cause a variety of errors including 506 errors.  So at a minimum, make sure that if you use anti-virus software or a live backup software such as Carbonite, that you configure these to exclude the Atrex data folders.
  

How long portions of the database are locked on the server for updates and how fast those locks are managed can make a big difference, so anything that reduces the lock duration will also reduce the 506 occurrences.  Looking forward, we have a few things coming down the pipe in v18 that will help reduce the 506 errors on the server side.  These include reworking of a few key functions in Atrex to reduce the time the database is locked when adding or updating transactions in the AR table, some new performance optimizations, and an updated version of the database engine that manages the server locks more efficiently.