LiPo Rider 1.3 - not charging?

Hello,

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?

greetings
MoP