Uhh, I'm sorry :-)
By now: chop the tree - and jump on it. Jumping may be important.
As you fall into the water jump out at the other riverside. This is possible too.
Or is most cases, as you reported...

I had a lot of work persuading the game to build a path over the tree.
There is a invisible wall inside the fallen tree. The zombies try climbing at it when the panic event is running. This is an error too, but you could locate it this way.
The fallen tree is also "caged" by invisible walls. The distance is that wide to ensure the passing. Thereby players are falling down again... I'm truly sorry!
(Funny to mention I made this event because I wanted a lazier solution. At first I wanted to create a build-a-raft but that seemed to complicated to me. So simple tree cutting does not seem to be that simple too...)

I am surely going to change this but I have to put you off until the weekend. Real life needs most of my attention until friday.
Thank you, and thank all of you, for reporting the errors!
Your different approaches help me a lot curing all these map diseases.