So I know I'm not alone on this issue, I've come across countless topics and I couldn't find a solid solution ANYWHERE and I tried EVERYTHING. I finally stumbled upon a solution that worked for me and it DIDN'T involve a clean wipe of my ROM.
(oh and a little background, I'm running a Droid 4 on Verizon with stargo's port of CM11 KitKat)
The topic is on the cyanogenmod website (can't post the link because of stupid forum rules), and I found the solution in the comments. Here is the comment by Matt Mower:
"Michael Gernoth, after changing apns-conf.xml, you might need to force a reload of apns-conf.xml. One way to achieve this is:
1) Enable airplane mode
2) Restart phone
3) Settings > Mobile networks should now show "Access point names" as a menu you can enter
4) In Access point names menu, choose "Reset to default" from the drop menu
5) Disable airplane mode"
So I did that, and wallah, I was able to send and receive MMS again and the error is gone!! I've seen soo many other people on here who have the same issue and have tried to reset to default and they mention that it wouldn't save, and I think the reason is it just needed that said manual reload of apns-config.xml.
Please note that I take NO CREDIT FOR THIS FIX, I just happened to find the post that contained the info that I believe many others have missed. Please share this info with anyone else having this issue as it could save them a ton of frustration as well.
Thanks for reading!