Thanks everyone. I can reproduce it now, even without ubuntu-dock. Looks like it is:

  https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3969

but there are two flavours:

----- This is "normal" when opening the overview ----

May 24 16:15:02 kab gnome-shell[5789]: value "nan" of type 'gfloat' is invalid or out of range for property 'x' of type 'gfloat'
May 24 16:15:02 kab gnome-shell[5789]: value "nan" of type 'gfloat' is invalid or out of range for property 'y' of type 'gfloat'
May 24 16:15:02 kab gnome-shell[5789]: JS ERROR: Exception in callback for signal: shown: Error: Wrong type number; float expected. But it's out of range: -Infinity
                                       _init/GObject.Object.prototype.set@resource:///org/gnome/gjs/modules/core/overrides/GObject.js:553:16
                                       _easeActor@resource:///org/gnome/shell/ui/environment.js:155:15
                                       init/Clutter.Actor.prototype.ease@resource:///org/gnome/shell/ui/environment.js:294:19
                                       _syncWorkspacesActualGeometry@resource:///org/gnome/shell/ui/workspacesView.js:728:21
                                       _init/this._overviewShownId<@resource:///org/gnome/shell/ui/workspacesView.js:396:18
                                       _emit@resource:///org/gnome/gjs/modules/core/_signals.js:114:47
                                       _showDone@resource:///org/gnome/shell/ui/overview.js:604:14
                                       onComplete@resource:///org/gnome/shell/ui/overview.js:589:36
                                       _makeEaseCallback/<@resource:///org/gnome/shell/ui/environment.js:85:13
                                       _easeActor/<@resource:///org/gnome/shell/ui/environment.js:168:64


----- This appears to be the bug(?) -----

May 24 16:15:07 kab gnome-shell[5789]: value "nan" of type 'gfloat' is invalid or out of range for property 'x' of type 'gfloat'
May 24 16:15:07 kab gnome-shell[5789]: value "nan" of type 'gfloat' is invalid or out of range for property 'y' of type 'gfloat'
May 24 16:15:07 kab gnome-shell[5789]: JS ERROR: Error: Wrong type number; float expected. But it's out of range: -Infinity
                                       _init/GObject.Object.prototype.set@resource:///org/gnome/gjs/modules/core/overrides/GObject.js:553:16
                                       _easeActor@resource:///org/gnome/shell/ui/environment.js:155:15
                                       init/Clutter.Actor.prototype.ease@resource:///org/gnome/shell/ui/environment.js:294:19
                                       _syncWorkspacesActualGeometry@resource:///org/gnome/shell/ui/workspacesView.js:728:21
                                       animateToOverview@resource:///org/gnome/shell/ui/workspacesView.js:577:18
                                       animateToOverview@resource:///org/gnome/shell/ui/viewSelector.js:278:33
                                       _animateVisible@resource:///org/gnome/shell/ui/overview.js:582:27
                                       show@resource:///org/gnome/shell/ui/overview.js:568:14
                                       toggle@resource:///org/gnome/shell/ui/overview.js:691:18
                                       _initializeUI/<@resource:///org/gnome/shell/ui/main.js:238:22