we are getting the error: “ERROR: 10 - [NoFieldFound]No field found: Contact.inf_custom_xxxxx”
but the field inf_custom_xxxxx definitely exist and same API did work before.
It looks like you might have a question regarding the usage of an SDK, rather than the API directly. I’d recommend you reach out to our Advanced Support team, who can possibly troubleshoot with you more directly using your tenant instance configuration.
Yea! Sorry for not posting here 
In fact I contacted support and they were very helpful.
Short version: I need to use _XXX field names (starting with underscore) an not inf_custom_XXX
Just like normal fields.
So it is now resolved. However it’s still a mystery hod it worked for years for us with the prefix