LiPo Rider 1.3 - not charging?


I have the

  • LiPo Rider 1.3
  • 2200mAh 3.7V Batterie
  • 3W Solar Panel 138X160 (POW92136O)

It seems that it is not charging - Example (no direct sun, but only a bit cloudy):
the red LED is shining brightly.

Solar Panel disconnected: 5,5 V
Solar Panel when connected to LiPo Rider: 4,61 V
Battery connected only has: 3,78 V (when sun comes out, it goes up to ~ 3,81)
Battery disconnected also 3,78 V

When connecting the micro USB charging cable, it is charging at 3,91 V, which looks ok.

have you got some idea, what could be wrong when charging solar only?