I can confirm that this is resolved in 5.0.0.2167.

Tracked as 15552.

This is unchanged in 5.0.0.2166.

BTW, I can avoid having to use the Task Manager when I select to restart MM5 in safe mode.

I can confirm the behaviour when editing the exended tags. I cannot create a new one nor edit an existing tag. And like Snofru i had to kill the entire mm5 with the task manager.
MM5 beta 2164 here too.

Not seeing this here in version 2161. added new field exactly as followed. I had no previous entries in the existing Custom fields

MM5.0.0.2164

Crash recipe:

Open properties from any song.
Select “Custom”.
Klick on “+” to add a new field.
Press OK or Cancel – MM5 crashes.
Crash log C62E4E2D followed by C49191E2 and C62E2A49.
MM5 had to be killed by Task Manager.