Serger/overlocker problems! Help! Sewing Machines Parts and Sales in Los Angeles

Hi sewing brain trust! I need your help.

I recently re-threaded my serger (Janome MyLock 744D) with a new colour. It’s not the first time I’ve done it but I am pretty new to serging. I initially re-threaded ‘the easy way’ by cutting the threads above the tension dials, changing the bobbins, re-tying the new threads to the old ones and then pulling them through and threading the needles. This is the recommendation in the manual. Shortly after I started sewing, the upper looper and lower looper threads snapped so I obviously did something wrong.

I got my manual out and un-threaded then re-threaded everything from scratch in the order shown. This time when i pressed the foot pedal without any fabric the threads became a chain and all was well but about 2 inches into sewing my test fabric the upper looper thread snapped.

I went around this loop 2 more times and the same thing happened. Always the upper looper thread.

Has anyone had this happen to them? Any ideas on what I’m doing wrong? Thanks in advance.