For me it was some :title and :subtitle attributes (thanks @Kinco) and an issue with the v-text attribute as well. To summarize:
INVALID:
<v-toolbar-title
v-text="header" />
WORKING:
<v-toolbar-title>
{{ header }}
</v-toolbar-title>
Using Node 22.9.0 (runtime) and Bun 1.1.20 (builder).