I was getting a "Maformed Response" error when doing a few things this week. I saw it from the admin page when adding a new contact and when merging duplicate contacts. Other users saw it when making a contribution.
Error message displayed was:
Sorry, due to an error, we are unable to fulfill your request at the moment. You may want to contact your administrator or service provider with more details about what action you were performing when this occurred. Malformed response Return to home page.
Fatal error: Uncaught CRM_Core_Exception: [0: Transaction integrity error: Expected to find active frame thrown in /PATHREMOVED/public_html/administrator/components/com_civicrm/civicrm/Civi/Core/Transaction/Manager.php on line 107
I first cleared out the cache (templates_c directory) without success. I then went into the control panel and removed the mapping and geocoding providers (I was using OpenStreet maps and Yahoo). This seems to have fixed the problem.
Running civicrm 5.10.3 under Joomla 3.9.4.
I'll report back if I see the problem again, but I think its all good now.