Can't submit a message to an existing conversation

[ontheriver]

I've been trying to send a confirmation message that I received the item but keep getting the same error message. I've been able to send a message to a different member. Not sure if this user is blocking my messages or something. My message doesn't seem inappropriate or "spam-like":

 

[Spark]

Change the wording a bit and see if it works, seems like it's getting caught in the spam filters

 

[ontheriver]

I did try a few different versions but got the same error. Since I was able to send similar message to other sellers I thought maybe that’s a message if they were blocking me or something. Not a big deal since I was able to leave feedback. Hopefully just a glitch. Thanks for responding.