[SOLVED] Vtiger 7.2 query operations to REST API returning 500 errors

I've recently upgraded from Vtiger 6 to 7.2 (a clean installation) and all my requests to the REST API that use the query operation are no longer working. It doesn't matter which module the request is for e.g. Contacts, Leads, Accounts. All other types of operations are working e.g. retrieve, describe, but a query such as select * from Contacts where email = 'foo@bar.com'; will fail with a 500 Internal Server Error returned from the Vtiger server.

Here's an example of my HTTP request (query param is left unencoded for readability):

https://crm.myendpoint.com/webservice.php?sessionName=[mysession]&operation=query&query=select * from Contacts where email = 'foo@bar.com';

The code I'm using to make my queries is completely unmodified from when I was using version 6 of Vtiger and the requests were working fine then. I've switched on debug logging on the server but there's no errors.

The server is receiving and processing the request though. At one point, it dumps the data to the log for the Contact that I'm querying (which all looks correct), and then here's the last few lines of the logging before it ends:

Mon Jan 20 17:13:41 2020,292 [8010] DEBUG webservice - Entering isPermitted(Contacts,DetailView,) method ...
Mon Jan 20 17:13:41 2020,292 [8010] DEBUG webservice - Entering getActionid(DetailView) method ...
Mon Jan 20 17:13:41 2020,292 [8010] INFO webservice - get Actionid DetailView
Mon Jan 20 17:13:41 2020,292 [8010] INFO webservice - action id selected is 4
Mon Jan 20 17:13:41 2020,292 [8010] DEBUG webservice - Exiting getActionid method ...
Mon Jan 20 17:13:41 2020,292 [8010] DEBUG webservice - Exiting isPermitted method ...
Mon Jan 20 17:13:41 2020,293 [8010] DEBUG webservice - Entering getColumnFields(Accounts) method ...
Mon Jan 20 17:13:41 2020,293 [8010] DEBUG webservice - in getColumnFields Accounts
Mon Jan 20 17:13:41 2020,293 [8010] DEBUG webservice - Prepared sql query being executed : SELECT tabid, fieldname, fieldid, fieldlabel, columnname, tablename, uitype, typeofdata, presence
        FROM vtiger_field WHERE tabid in (?)
Mon Jan 20 17:13:41 2020,293 [8010] DEBUG webservice - Prepared sql query parameters : [6]
Mon Jan 20 17:13:41 2020,293 [8010] DEBUG webservice - Exiting getColumnFields method ...
Mon Jan 20 17:13:41 2020,294 [8010] DEBUG webservice - Entering getColumnFields(Accounts) method ...
Mon Jan 20 17:13:41 2020,294 [8010] DEBUG webservice - in getColumnFields Accounts
Mon Jan 20 17:13:41 2020,294 [8010] DEBUG webservice - Exiting getColumnFields method ...
Mon Jan 20 17:13:41 2020,294 [8010] DEBUG webservice - Prepared sql query being executed : select 1 from vtiger_crmentity where crmid=? and deleted=0 and setype='Accounts'
Mon Jan 20 17:13:41 2020,294 [8010] DEBUG webservice - Prepared sql query parameters : [9637]
Mon Jan 20 17:13:41 2020,294 [8010] DEBUG user - Entering Users() method ...
Mon Jan 20 17:13:41 2020,294 [8010] DEBUG webservice - Entering getColumnFields(Users) method ...
Mon Jan 20 17:13:41 2020,294 [8010] DEBUG webservice - in getColumnFields Users
Mon Jan 20 17:13:41 2020,294 [8010] DEBUG webservice - Exiting getColumnFields method ...
Mon Jan 20 17:13:41 2020,295 [8010] DEBUG user - Exiting Users() method ...
Mon Jan 20 17:13:41 2020,295 [8010] DEBUG webservice - Entering getColumnFields(Users) method ...
Mon Jan 20 17:13:41 2020,295 [8010] DEBUG webservice - in getColumnFields Users
Mon Jan 20 17:13:41 2020,295 [8010] DEBUG webservice - Exiting getColumnFields method ...
Mon Jan 20 17:13:41 2020,295 [8010] DEBUG webservice - Prepared sql query being executed : select 1 from vtiger_users where id=? and deleted=0 and status='Active'
Mon Jan 20 17:13:41 2020,295 [8010] DEBUG webservice - Prepared sql query parameters : [1]
Mon Jan 20 17:13:41 2020,295 [8010] DEBUG webservice - Prepared sql query being executed : select groupname from vtiger_groups where groupid = ?
Mon Jan 20 17:13:41 2020,295 [8010] DEBUG webservice - Prepared sql query parameters : [1]
Mon Jan 20 17:13:41 2020,295 [8010] DEBUG webservice - Prepared sql query being executed : select first_name from vtiger_users where id = ?
Mon Jan 20 17:13:41 2020,295 [8010] DEBUG webservice - Prepared sql query parameters : [1]

I was thinking that this might be some kind of permission issue, but surely the server wouldn't return a 500 in that situation. In any case, I've tried running queries with 2 different users, both of which have an admin role.

This problem only happens with query operations, irrespective of the module being queried. Is there any way I can debug this further?