Glad you sorted it out. It is (touch wood) something that has never happened to me. Have had other problems but never a birds nest.
My Bernina will do that when there is even a tiny thread bit in the bobbin area. There are many other reasons that this could happen and I think they may be all listed below.
A nest underneath means a problem on top such as incorrect threading. Be sure to check and rethread. Hugs. Nan W
I just had this yesterday with my janome 500e. It does not like a bobbin to be wound completely full like my 350e. For some reason as the hook comes around the bobbin unwinds too many strands of thread then the cutter cuts all the strands of thread it has pulled out and voila! A big mess! I removed some thread from the bobbin and cleaned it all out again and it is fine again.
I have also had this happen when the needle is blunt and the stabiliser or thread is pushed down a little too far and the hook messes things up. So you could try a fresh needle too.
probably just the needle and/or the bobbin. It's a nuisance when the machine keeps going isn't it.
Really, I have no clue, but just a guess. Could there be a small stray thread in the bobbin area that may have moved into just the right position to catch the thread as it came through and cause this wonkiness? I shall follow this with interest as more knowledgeable folks post answers. Thanks.
Thanks for replying. At the weekend I took the machine apart and cleaned bobbin area and removed all the gunk that was there. This has happened before my clean up. it then stitched out fine after the mishap