This is a common and longstanding SpeedTree import crash that has occurred since at least 4.14 for over 100 users.
This seems to be the same issue as a previous Jira from 4.10 which was closed without repro steps, [Link Removed].
The real repro steps are currently unknown. It seems caused by importing free SpeedTree assets ([Link Removed])
That being said, we have a way to trigger the issue that is causing the crash that the users are experiencing.
1- Import the file SpeedTree_Test.srt found in the folder Engine/Content/NotForLicensees/QA/Import_Test_Assets/Partners/SpeedTree with the following settings.
2- Delete the asset SpeedTree_Test_Branches_2_Mat.
3- Rename the static mesh asset SpeedTree_Test to SpeedTree_Test_Branches_2_Mat.
4- Redo step 1.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-50033 in the post.