Reproduction:

Add a planar reflection probe to the default scene (also reproduces with regular ref probe)

Error:
Unable to load the icon: 'ReflectionProbe Gizmo'.
Note that either full project path should be used (with extension) or just the icon name if the icon is located in the following location: 'Assets/Editor Default Resources/Icons/' (without extension, since png is assumed)

Note: Adding a reflection probe on the same Unity without HDRP does not cause the issue.