This crash still happens if you summon a boat as a passenger of a boat that is a passenger of an entity.

Steps to reproduce

1. Insert the following Command into a CommandBlock and trigger it:

/summon creeper ~ ~2 ~ {Passengers:[{id:boat,Passengers:[{id:boat}]}]}

Happens also when you specify the boat material type:

/summon creeper ~ ~2 ~ {Passengers:[{id:"boat",Type:"oak",Passengers:[{id:"boat",Type:"oak"}]}]}

2. Result

• My world gets unresponsive, when I open the GUI of the CommandBlock, it shows up empty.
• It's as if I'm in a ghost mode, but I can make screenshots still though, successfully.
  See screenshot of the unresponsive CommandBlock GUI (buttons are not clickable).
• I seem to be able to place blocks, but when I relog into the world after I forcequit, they are not there.

Sadly I can not force a crash with F3-C, and I don't get any crash report when I forcequit, but in my Launcher it says this:

[08:03:32 ERROR]: Game ended with bad state (exit code -805306369)
[08:03:32 INFO]: Ignoring visibility rule and showing launcher due to a game crash

Reproducable 100% with that command for everyone, to confirm it.

Interestingly something possibly new I noticed - not sure if it never occurred in the older MC versions with only 1 boat as passenger or I just overlooked it:
When I summoned the ridden entity, crashed, and re-log into the world, the entity and its passengers are not in the world anymore.

If I recall correctly, that's different from before (with 1 boat as passenger), but I don't know if that really matters.

Thank you, regards,
Meri