I don't know how useful this report will be, but I do believe there is an underlying Spigot issue or potentially even a Minecraft issue that causes the server to crash when the MythicMobs plugin is running on the server. This happens with both the latest free version of MythicMobs (v.4.5.8) and the latest premium development build that got basic 1.14 support (excluding build 2778), running with default configs on a clean server.
v4.5.8 works great on 1.13.2 with default configs.

Steps to reproduce the crash:
1. Install the latest free version of MythicMobs (or the latest premium build)
2. Login and walk around, loading/generating chunks.
3. The crash will happen rather quickly, and only when MythicMobs is loaded. In this case i used /tps to see when the server stopped responding. /pl can be used too.

Edit: The crash is related to random mob spawning. In the latest development build (2778) of MythicMobs, random spawn points are removed which prevents the server from crashing.

I have generated a full thread dump that is includes the hanging state before spigot makes its thread dump. By having this as a start option: -XX:+PrintClassHistogram and then using ctrl+break once the server is unresponsive.

thread19.txt is what I think is the important part. logdump.txt includes everything.

I have talked with the MythicMobs developer about this very issue, and we think it might be a Spigot issue.