After having successfully published a score fr/Studio to musescore.com, shortly (~1/2 hour) later attempted another upload but was prevented, with an attendent pop-up station that it could not be published "at this time".
A few weeks before when this happened, restarting Studio corrected the issue, but not this time, and it's continued over a couple of days. No suggestions have been found on the .org forums.
Also, the same issue occurred on both Windows and Ubuntu Linux.