There seems to be a regression in Embedded Host Client 1.29.0
I cannot edit any VM anymore, either running or stopped with the same error message as OP.
Sys:
5.5.0 Update 3 (Build 7967571)
VIB:
esx-ui 1.29.0-8122819
VM Version:
10
Tried Firefox and Chrome, both have unhandled exception when editing VM Settings.
The old native Client works w/o problem.
Thx