Error messages in solving a system of ode's

I have been working with a system odes and have serveral questions about error messages.

I have been using Radau (system is stiff; solutions have sharp peaks). For a certain range of parameters I get the message "The return value of this function must match the problem size" referring to the vector the the rhs's. I circumvented the problem by switching to BDF. But what does this message mean? (I couldnt find it in the list of error messages.)

Then, I wanted to try Stffb and Stiffr. I had 10 equations and got the message "does not solve more than 8". Ok, I reduced the system to 8, but still got the message. Then reduced it to 7, thinking the message actually meant 8 or more, but got the same message. Then reduced it to 7 and I get the message "This value must be real."

Can anyone help? Happy Holidays!