Currently we do not support APEX 5.1 in any version since it has only just been released. In order to certify FOEX on a new release of APEX we require a downloadable version of APEX to install in our environments.
We will be releasing FOEX v2.2.1 in the near future to support APEX 5.1, and we are investigating whether it can be incorporated into FOEX v3.0.0 which is close to release, or whether it will be supported in FOEX v3.0.1.