Required string '{' missing

This is always due to a new pk3 you've downloaded having a missing brace ( } or {) from an .npc or .sab file. You should either contact the author or you can fix it yourself by opening the pk3, going to ext_data then either sabers or npcs. Open all the files in these directories and check them for a missing brace.

Here is an example of a working NPC file (Jawa)

jawa
{
playerModel jawa
weapon WP_NONE
scale 75
health 10
snd jawa
sndcombat jawa
reactions 3
aim 1
move 3
aggression 3
evasion 1
intelligence 5
rank crewman
playerTeam TEAM_NEUTRAL
enemyTeam TEAM_FREE
class CLASS_JAWA
yawspeed 90
walkSpeed 55
runSpeed 150
dismemberProbHead 0
dismemberProbArms 10
dismemberProbLegs 1
dismemberProbHands 20
dismemberProbWaist 0
}

Notice how there are two different types of braces before and after the personality instructions?

Fix

Check your .npc and .sab files in the pk3 you're having problems with and see if any braces are missing, if you find one then simply add it and save.