Hey there Grzegorz W,
Sorry for the late response ??
This error is most likely related to your permalink structure, please try setting permalinks to Post Name in Settings -> Permalinks or if this is already set try flushing permalinks (re saving settings).
If this doesn’t help please open a thread so we can investigate this further ??
Thanks for taking time to write a review and have a nice day!
Cheers,
Bojan