That... didn't make much sense, as my previous unit was running a newer firmware version, which would work fine with the LMS server, and the server software itself was way newer than the one on the SqueezeBox Touch.
Now, whenever you install LMS on the server, the installation will include suitable firmware. And the SqueezeBox (Touch) is supposed to recognize the LMS server, and download a newer version of the firmware.
Unfortunately, the older firmware on the second-hand SB Touch would not recognize my server, but it would immediately claim the server was running older software than the Touch. Euh... what? Logically, I assumed that the SB Touch firmware didn't recognize the LMS version properly, so I needed to update the firmware on the SB Touch. But how if it doesn't recognize the server where the update is located?
It turns out you cannot do that manually. (Maybe you can, but I couldn't figure it out.)
Symptom 1: Error message, asking you to update the server
Symptom 2: Settings / Advanced / Software Update only shows an older version.
Cause: the Touch is not (properly) querying the LMS server
Solution: Settings / Advanced / Diagnostics / Software update
Yes, at first sight this makes no sense. Why would a software update not see a new firmware, but a diagnostics would?
At second sight it still makes no sense... but it works, so mission accomplished 😅
(Some links suggest it might retrieve newer firmware from mysqueezebox.com, which is quite nice, but someday mysqueezebox.com will disappear... better update soon then).
It should be possible to update the touch using an SD card, but I haven't figured out yet how to.
I found some additional information here, just adding the link (and hoping the data will stay up until I've copied and added it to my own notes)...
The old wiki has a page for the old SqueezeBox controller. Still looking for an equivalent for the Touch, if there is one...