Yeah hard to say when the magnet is or could be effecting it as the machine does not send back information continuously. Have you cut any bed hold down pins? I would suggest trying those and then test out the machine on a scrap piece of paper on low settings to see if the error flags up again. As an example of how infrequent the information is sent back i had my white ribbon cable detach from the laser head (didnt listen for the click) and it took close to 3 mins before the machine actually shut down.
Here is a link for the pins Here
And a link in the post to where i posted a modified version of the design that i now use Here