@RD, your English is perfect. Most of us don’t speak 2 or more languages, so you are doing great.
@johnny, you have done a great job at troubleshooting. Considering all the strange results i would recommend a simple approach to start at the input and work to the output.
0. Remove all the goo and glue around the connectors and switch and any other place in which the wiring solder joints are covered. Inspect and touch up any cold or broken solder joint. Replace wires if necessary to ensure good path and eliminate the wiring or joints as a culprit.
1. Slice the blue film so you can verify the configuration of the cells, to measure and compare the individual cell voltages (looking for laggards), to load test and check capacity of individual cells, and to look for some sort of BMS board for the cells.
2. Rebuild the pack (replace defective cells) and BMS as necessary to eliminate that as a culprit of the problems.
3. Charge the pack and measure/verify the voltage and current while charging, if possible. Measure the pack voltage after charging to verify that the charger is working properly, no over or under charge, cuts off at limit, etc. Eliminate the charger as a culprit. [and like RD said, don’t try to run any load from a charger, not designed to work that way and possible to damage something.]
4. Test the driver and LED with this known-to-be-good pack. If the results are flaky with good solder joints, wiring and pack, then the driver is suspected as a culprit. Repair or replace.
5. Now the light has been rebuilt from front to end, and should give good service, especially considering the price.
Here is just an example of some various drivers: open boards, enclosed, and waterproof.
Good Luck and Enjoy.