SOLVED: after restarting the game, the undead vendor was autmatically attacked by the apothecary and so I could join in defeating the enemy.
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Hi there,

a walking dead was attacking the town (soulseeker quest) and I failed to be in town quick enough to kill it.
As a consequence several towns people turned into undead persons, including a vendor standing nearby questgivers.
Now i cannot talk to those quest givers anymore as the game says

"enemy to close".

ThusI I cannot finish my already completed quests and cannot get new ones from actually 2 quest givers.

So how to kill the undead vendor? the usual attacks do work but do no harm (the games give also no indication a fight is happening or so).
For me this looks like a bug, currently, unless somenbody else has an idea....