Update: replaced the DC-DC converter 48-24V over a week ago, and I didn't hear the costumer anymore. So probably this solved the problem. Thanks.

beste frids je stuur stroom daalt onder de 10 volt na lang gebruik
heb jij al je koel Finnen getest volgens mij is er 1 die erg zwaar loopt
groetjes michel

For me it looks like the problem with the power unit A1 (this is the big module linked with the traction motors).There is the set of unipolar transistors + lots of other electronics on top
Hope I'm wrong, because it's pretty expensive part.

Perhaps you have some other 335-02 truck to "borrow" A1 and to check it.

Of course, I presume, that the battery is OK, fully charged, no short circuits etc.