I very much like UB. But I really miss a working taskbar, as it hasen’t been with 18.04.
The panel used as taskbar overextends, so that the icons and a part of the taskbar were not to seen.
Is this problem solved in 19.04 or will it be solved or how can I solve it?
No it hasn’t been resolved. Upstream themselves do not recommend using the taskbar - instead upstream efforts have been mostly about resolving usability issues around the icon-tasklist - that is their recommendation to use.
You’ll also see a recent post where another user is using tint2 which has a traditional taskbar implementation.