Yeah, if you are getting that error from a "universal" (supposed to work on client and server) mod then the mod was improperly coded. The server doesn't have those methods/classes so will error out if the mod tries to access them.
Permanent Generation space is kind of like internal storage for classes and objects for the Java virtual machine. Java versions less than 8 required increasing the reservation for that memory beyond the default when there were lots of classes and objects (i.e. when there's a lot of mods). Java 8 eliminated permanent generation space and integrated that back into the normal memory allocation so you would be able to simply allocate with -Xmx and -Xms in order to reserve enough RAM.
Java.lang.OutOfMemoryError: PermGen space
Try adding the following arguments to your minecraft start. the default 128m usually are to small when using lots of mods. -XX:PermSize=256m -XX:MaxPermSize=512m