[Milhouse]

Code:

<pathsubstitution>
        <substitute>
        <from>special://profile/Thumbnails/</from>
        <to>smb://Stream-PC/kodi/userdata/Thumbnails/</to>
        </substitute>
    </pathsubstitution>

If you're sharing that directory between multiple Kodi clients, that is not supported, doesn't work, and is bound to fail. Remove it.

However, I am not sure since special://profile/Thumbnails/ should be universal for all Kodi installs.

Universal, yes, but intended for use by a single client only.