This other thread(linked below) has a lot of good details about this that can help guide you.
Are you saying the SlicerDcm2nii extension is unavailable on the extensions manager? Or after you install it, it doesn’t function/you can’t use it? It might be this issue related to the time you check for the extension.
As it relates to SlicerDMRI, it looks like code maintenance on it hasn’t been that frequent over the past year so there is likely a good chance that extension could be broken or breaking other things.
On Windows there are build errors building a test which look familiar to what me and @Sam_Horvath ran into during the transition to using an updated Visual studio toolkit. We ended up just disabling the tests which could probably be done again in the short term to fix the build issue.
@lassoan might know more about this topic, but you can disable it by pasting this code in the Slicer python console and then restart.
This means that DCMTK toolkit’s DICOM store SCP process (storescp.exe) is running. If you haven’t started it and you haven’t installed any other software that uses it then most likely 3D Slicer started it, because you enabled it in the DICOM module. In Slicer-4.11, you can enable/disable it by clicking on “Storage listener” checkbox in DICOM module’s networking section.