What are the inconsistencies? - you probably want to diagnose the exact cause of the problem first before taking other actions and see if you can simply fix that.
As part of the upgrade you will have most likely accepted Yes to make a backup of the working directory. This backup can be restored returning you to the old version.
You can untar that backup and copy the whole backed up nmis directory and contents in over the top of the current one thus replacing the 8.6 version with 8.5.6 version.
You MUST consider if during the upgrade you have made changes to other items outside of the nmis directory e.g. done changes to rrd database directory structure, moved from .nmis files to .json files for nodes etc. . Of course you should obviously backup the current 8.6 before doing so. You will have to consider carefully what has been changed on the system since the upgrade to make sure everything is consistent to when the back up was taken.
this is what alex zangerl said. " however, it looks like your nmis is configured inconsistently: the setting for '<nmis_base>' in conf/Config.nmis says '/home/nmis' , but you're running your programs out of '/usr/local/nmis8'. running nmis outside of /usr/local/nmis8 is not a currently supported configuration - in general it should work, but you're on your own. check whether /usr/local/nmis8 is identical to /home/nmis. if not choose one, ditch the other and make it into a symlink."
i created symlink but didnt resolve permission errors.
lrwxrwxrwx 1 nmis nmis 15 Dec 15 10:51 nmis -> /usr/local/nmis