Either the latest git version or my - very rare - cold reboot solved it.
I don't know, but the errors are away :-)
** Changed in: mesa (Ubuntu)
Status: New => Invalid
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Posted the bug to https://bugs.freedesktop.org/show_bug.cgi?id=110849 as
well.
** Bug watch added: freedesktop.org Bugzilla #110849
https://bugs.freedesktop.org/show_bug.cgi?id=110849
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