With default settings, the Nextcloud client may be killed off due to trying access too many of the synchronized files immediately upon opening. It depends on how many files are in ~/Nextcloud.

Unless the client code is changed, the only (inelegant) way to solve this is to increase the openfiles-max and openfiles-cur settings in /etc/login.conf. I chose to make the change for the staff class; add the following entries to the class:

:openfiles-max=10240:\ # default is 1024
:openfiles-cur=10240:\ # default is 512

Yes, this is pretty ridiculous, but when the client apparently tries to access every single file in the folder at the same time, there isn't much I can do.