For anyone encountering this in the future, what seems to have worked is to use the shortcuts for "save as" (option + command + s). Thanks to a reddit user for pointing this out.
For some weird reason, by using the shortcut to open the save as dialog box in AC26 as opposed to mouse clicking "File" then "Save As", the shortcut seems to have unstuck the save dialog in BOTH AC26 and AC25 at the same time. Now it works in both programs.
Aaand the bug is back... Using the shortcut to save might have unstuck the save dialog only temporarily. Also, there could have been a confounding variable to this, as I was using archicad via VPN yesturday. Though I did try it now at the office, using the same VPN (that we use to acess the license server for remote work) - but to no avail.