undo-tree problem after updates

Hi!

I often install a new emacs git version.  After many (all? not quite
sure) of these updates, the first start of emacs is unusable because
as soon as I try to enter ESC-x, I get this error:

Symbol’s value as variable is void: undo-tree-mode--set-explicitly

Any normal letters work, 'ESC' alone also works and shows me the
possible options for the next letter, but every command I tried ends up
with this, including ctrl-x ctrl-c.

Killing emac and restarting it gets rid of this, so it's a first-start
issue only.

use-package list from my startup file, in case it matters:

bison-mode
cmake-mode
consult
corfu
corfu-terminal
doom-modeline
eglot
embark
embark-consult
flymake
jinx
json-mode
leuven-theme
lua-mode
magit
magit-delta
marginalia
markdown-mode
modus-themes
nerd-icons
nerd-icons-completion
nerd-icons-corfu
nerd-icons-dired
orderless
php-mode
rust-mode
savehist
undo-tree
vertico
wgrep
which-key
yaml-mode

Does anyone have an idea what could be the problem here?

Thanks,
 Thomas