Hello Arnulf,
You will need to check to see if the battery is heating up.
It could be that the charger is trying to charge the battery, but the battery is not accepting the charge. This causes the battery to heat up and vent. It also causes the charger to heat up as well.
Charge the battery for an hour, or so, and remove it from the light to see if it is heating up. While you have it out, check the voltage.
Gel cell batteries really don't care what position the battery is in and will work fine on their side or upside down. However, when they wear out they heat up during the charge, and if the charger doesn't shut down they can vent.
Do some more checking and report back and let us know what you find out.
Tom