Allow connections from a remote server in MySQL

Assuming the permissions are as you say, it should work - however its worth noting that GRANT provides an interface to the underlying access control tables in MySQL - and for a long time it was standard practice to write to these directly; have you checked what is in the mysql.user tables?

Have you checked that the client address seen by the server is the same as you specified in the grant? i.e. 50.xx.xx.xx is the only address for sandbox.abc.com

Also, there is note in the mysql manual regarding this error:

On Linux, another reason that this error might occur is that you are using a binary MySQL version that is compiled with a different version of the glibc library than the one you are using