Because Plasma 5 widgets need to be ported to work with Plasma 6, all unported widgets currently installed on users' machiens will be broken and show the typical "Sorry! There was an error" text. This is technically accurate, but probably not an ideal UX. We should try to detect when a widget is un-ported and show a more user-friendly error message so that the user understands what's going on.
now the error message tells that the widget was done for an older incompatible plasma version
Fixed by Marco Martin with https://invent.kde.org/plasma/plasma-framework/-/commit/d2b88b2d56963e4b60f0dc1700be8a8eebc68e80 for Plasma 6.0