Hi Kevin,
I know how frustrating something can be when something does not work 🙂
Let me explain the situation a bit more clearly and what i think is going on.
When it does not return the proper preview the server is returning the code 406 (Not Acceptable) instead of 200 (OK), this seems to be a server level error. We have seen similar things in the past with certain names for fields and in most cases the is a mod_security rule on the server, mod_security is a sort of server firewall that has many security rules that prevent actions if it thinks it might be a hack attempt. I myself have had to disable many rules for customers on my server over the years because of false positives.
Now your server admin should be able to looks at the server logs and it should tell him something like mod_security rule 12345678 triggered at date XYZ, he would then go and disable that rule 12345678.
To test this theory, you can try changing the names for any custom fields u are having problems with to something generic like “field1, field2 etc…” (though the server might be blocking the “http” part of the value).
In short this is not something we can fix, it needs to be fixed by a server admin (try googling server code 406 mod_security)
Please let us know how you get on.
Thanks,
Stiofan