Tvheadend

I managed to configure TVH such that the EPG work for all my channels (DVB-T2 HD German channels w/o encryption, e.g. no privates). When hitting teh "Watch TV" button, TVH is loading for about 10 secs before the TV picture appears. It shows, however, just some series of stills, not a viewable video. Its not just dropouts, its really stills.

The Raspi 4 decodes H.265 in hardware, should therefore have no problem with HD TV at all. Either my configuration is bad, or the TV Hat is broken.

Is there any advice? Thanks in advance