"fork() failed" is due to the limit of processes allowed for VPS has been reached.
If you use default LSWS configuration, it is not likely cause that problem. however, if you have tune some configurations to a high value that beyond what you VPS can handle, it could cause some problem.
You need shell access to check how many processes has been started, "ps -ef" should tell you that. if you have too many processes of certain type. you need to figure out why.