Does the forum logs give an error when it crashes?
Do you have mongo and nginx logs on?
There should be some useful information in one of the logs as to what is happening.
In NodeBB 1.18.6 ACP, when I click;
ACP > Dashboards > Logins or Users or Topics
the page URL changes to include one more /forum, additionally, this subfolder doubling "/forum/forum" can be seen in the links on the ACP page: (such as last 30 days/ last 7 days on the image below)
This problem only occurs in Logins/Users/Topics pages, not in "Overview" or "Searches"...