Agrees - low belt tension.
But also check for alignment, pulley flange damage / bent ?
Loose rear joint (bolt from RHS / nylon washer between bottom tube and rear ‘part’ ? … causing poor rear pulley alignment (but I’m sure you notice this amount of ‘looseness’).
Problem with Front bottom bracket bearing - loose ?
F pulley not aligned on crank
F Pulley cracked or warped in heat ???
Now I’m stuggling to guess any more 
good luck and please let us know what is the cause of this very weird happening.