When I try to join my own vanilla hosted server (keep in mind it has worked very well in the past where I and others could join, and YES I have tried deleting the player data files) it DOES NOT allow me to join back regardless of the attempts of fixing it ive done. It is strictly vanilla on the latest 1.13 pre-release 5 snapshot.
This has happend for others on main versions of minecraft as well. This IS solvable but its just that I don't know the solution, thats why I posted here. The problem also Is that after much more searching I still have not found away to contact mojang support, and it also seems like this issue might be the first of its kind for this version meaning either nobody has had this error for this version or if they have, they found a way to fix it in a way I havn't found so far.
Mojang does not have a technical support team. Snapshots and prereleases ars not meant for general gameplay, they are test versions for the developers to get community feedback. If you have issues with prereleases report it on the bug tracker.
If this issue occurs in a stable release then post the server log of that version.