Messerli & Kramer Threatened me with garnishment when I had an active auto pay payment plan running St. Paul Minnesota

I have a payment plan set up with Messerli and Kramer on a very old credit card debt. The line of credit was $750. The current balance is in excess of $8k. This bill is in at least two decades old. 

M&K helped themselves to my bank balance roughly a year ago. I was able to get the funds back, and went on a payment plan to prevent such a thing happening again. The payments were taken automatically, the Monday after I got paid every two weeks. Prior to that it was $75/mth when I got paid 15th and end of month. I have never asked for the payment to be held, nor has the payment ever bounced. This last payment never came out. When I noticed that, I called my case person and told her to keep the payments going, the money is still there for the missed payment and take the next one, etc.

Today I received legal paperwork stating that they intend to garnish my wages. They filed through the wrong county - I don't know if that will save me, though. I left a very angry voice mail on my rep's phone, and intend to call again first thing in the morning. I don't know how it's even legal to go ahead and help yourself to someone's wages when they are doing exactly what they are supposed to do. I didn't miss that payment, they did. I operated fairly and honorably; they did not. 

I am so very angry.