CB Login Module JSON HTTP 500 Error (Login not recognized by DT Register)

Since upgrading to CB 2.1, experienced problem where although the CB login works OK, the login status is not recognized by DT Register as it was before (Joomla 3.6.5, DTR 3.1.13) so users can't renew registration as they are prompted for a new account registration which errors out as "user already exists. The CB Login Module is the latest version. In the Joomla Module Manager it shows an error "JSON Error HTTP 500." (see attachment) which suggests that it is not communicating login status. Appreciate any troubleshooting recommendations.

Not sure what to tell you.
We have tested our login module with Joomla / Community Builder - not DT Register.