To reset the known U/P credentials for ALL devices
  Settings Tab -> Settings -> Config Collector
  Click the "Reset All Authentication Info" button

To reset the known U/P credentials for a single device
  Open the Device Viewer for the particular device
  Select Config Files -> Settings
  On this Screen, click the "Reset Authentication Info" button


This will force NetMRI to attempt to guess the credentials for this device, or all devices, again, based on the credential input to the Global list - Settings Tab -> Settings -> Config Collector

I get lots of Issues that say that my Enable Password is WEAK...
Since I'm new to NetMRI I don't really know how to deal with that. I don't want to simply suppress this Issues. I would like to find a solution.

Maybe you can help me?

A device will be cited for a "weak" password if the password being used is one of the known defaults for that specific device. A cisco device will also be cited for a "weak" password if there is  no enable password configured. 

A lot of customers do not use enable passwords but instead configure users with enable privileges.