We have never heard of that happening before.
First, make sure that you are using the final version of NavXtender 1.4.7786 Client and have the correct 32-bit or 64-bit version to match your NAV version.
Next, check that you don’t have any anti-virus, malware, or other security applications that can interfere with file editing:
You can also try using a local config file instead of a network config file, or vice versa.
The last suggestion would be to change the config file properties to Read-Only to prevent additional changes to it.