[horde] Jessie Apache 2.4.10 error - Global Preferences - data not passed in post or get

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I have horde groupware 5 webmail edition latest stable running fine on  
debian jessie with apache 2.4.10, and php 5.6.12, with one important  
exception - when logging in as superadmin, I cannot edit any of the  
Global Configuration settings - under Administration -> Configuration   
- I receive an error that no data was sent. err_empty_data

for example the url /horde/admin/config/config.php?app=horde gives me  
this errror message


I also cannot reach /horde/admin/user.php - same error.

I wonder if this has something to do with the apache2/php.ini settings of
enable_post_data_reading
or
always_populate_raw_post_data   both of which I have left commented  
out (playing with various settings here did not solve the problem; I  
have read that for the latter , apache 2.4 wants it to be set to -1,  
but that did not help for me.)

Is this a known problem ?
Any help on where to look?

Thanks.

ed