After the dissolving process from the coupler (VKUP) I can hear a quietly fizzling, coming from the connector plug (VTEK)! Is my connector plug leaky?

No! There is a built-in reverse-flow brake in the connector plug, which prevents a too quickly leak from the air in order that the dissolving process isn´t too strong! After the dissolving process there´s a residual air between reverse-flow valve and your nonreturn valve. This residual air leaks slowly because the locking pawl in the reverse-flow valve does not seal but slows down the air flow. If you can hear this fizzling for a too long time, please check your nonreturn valve.