When I plug a removable device, sometimes clicking "open this removable device" just has no effect whatsoever for no apparent reason.

When it does open the removable device it uses konqueror instead of dolphin. Where is the setting to have the removable device systray icon use dolphin as expected ?

Check what application is handling the file management in Applications -> Default Applications from systemsettings5.
If it does not help, from Applications -> File Associations check which application handles the MIME type inode/directory.

I'll have a look on the affected computer tomorrow morning. I'm pretty sure that the default application is dolphin but I don't know about the inode/directory file association.

I'll let you know tomorrow, thanks for the pointers.

I had uninstalled konqueror as a temporary workaround and upon reinstalling it silently sets itself as the default file manager so I can't replicate the state the computer was in.

It seems wrong that installing konqueror would mess customized settings.

I think konqueror was installed as a dependency and it comes with dolphin4, so I'll just remove konqueror altogether as a definitive workaround.

Thank you for your kind help.