Found the solution here:

https://www.gog.com/forum/divinity_series/resistance_bug/page2

Highlighting here the solution (by @PitBrat):

Okay, here is more info on how to fix the Evil Imps Bug in Divine Divinity.
This fix will prevent Zaknadrix and Antx from becoming hostile and attacking instead of speaking with the hero.
A simple hex editor is required.
In the Divine Divinity 'savegames' folder, locate a save just before confronting Zaknadrix.
Use Hex Edit to open the data.000 file
Search for the text 'Zanadrix' (notice there is no 'k' in the name)
Ignore the first match and look for the second and third matches which look like this:
ASCII = "Zanadrix59.....Zanadrix.K"
HEX = "5A 61 6E 61 64 72 69 78 35 39 00 09 00 00 00 5A 61 6E 61 64 72 69 78 00 4B"
Overwrite the hex value for both Z's with 00 so the string now looks like:
ASCII = ".anadrix59......anadrix.K"
HEX = "00 61 6E 61 64 72 69 78 35 39 00 09 00 00 00 00 61 6E 61 64 72 69 78 00 4B" Now go back to the top of the document and start another search for "Antx47"
Ignore the first match and look for the second and third matches which look like this:
ASCII = "Antx47.....Antx.K"
HEX = "41 6E 74 78 34 37 00 05 00 00 00 41 6E 74 78 00 4B"
Overwrite the hex value for both A's with 00 so the string now looks like:
ASCII = ".ntx47......ntx.K"
HEX = "00 6E 74 78 34 37 00 05 00 00 00 00 6E 74 78 00 4B"
Save the data.000 file
Run Divine Divinity, load the modified save and play!
Now both Zaknadrix and Antx are friendly and will speak to the hero allowing for the completion of the Council of 7 quest for the Imp.
Enjoy!
-PitBrat