@scmint also CRUCIALLY, verify the voltage between VOUT- and VOUT+ and make sure it's not higher than 5V before you plug those into BelaMini. The trimpot on the boards seems to be used to adjust that voltage across a very wide range.
Also, check with a multimeter while everything is unplugged whether the VOUT-, VIN-, BAT- and the USB port's case are all connected together.