I read the answer to this somewhere but I can't find it here. What do you do when your henchmen say they serve another master now?

Also, what is "Take 20"? why does it happen almost all of the time with my theif?

Try dropping any henchmen in your party and then try to rehire them. That might solve the problem. Either that or it's a glitch caused by importing a character into a single player game which has been converted into multiplayer. I've seen the phrase before - just a little vague on the details of the cause.

"Take 20" occurs when your thief performs an uncontested skill check, such as opening a lock or setting / disarming a trap outside of combat. This is equivalent to rolling a natural 20 on a skill check without actually rolling the skill check. The feat SKILL MASTERY allows a thief to "take 20" even in combat.

I think the henchmen not wanting to rejoin you has somthing to do with you not having the item they gave you when they initially joined you... Daelan's feather, Linu's token, and Sharwyn's contract come to mind.

You can Take 20 on other skills than thief skills, such as the Heal skill.

Make sure you have their contracts with you if you drop them and try to rehire them. That is one of the common causes of getting the "I'm working for someone else now" statement

And here's a basic way to explain taking 20... When making a skill check, a die roll is made. Taking 20 is equal to the player saying to the DM, "My character is willing to keep trying this until s/he gets the best possible result." So rather than wasting everyone's time around the circle rerolling dice, and assuming there's nothing (like a monster or pressing time needs) stopping the player from sitting for hours picking a lock, the player takes 20 and everyone's happy.