You (or someone) probably had the actual central model open and not just a local copy of it. You cannot have worksets without making a model a central model (FYI, so if there are worksets its a central model). Once someone converts a local model to a central model, the model should immediately be saved + synced (THIS IS A VITAL STEP MOST PEOPLE DON'T DO = ISSUES), CLOSED, and then reopened, forcing it to open a local copy. if not, the central model belongs to the person who has the actual central model open (not a local copy) and nobody will be able to sync or save to the central model. The error lies with whoever opened the file through the file browser and not through revit's "open" dialogue. If you open a central model through the file browser, it opens the actual central model and doesnt create a local copy. Alternatively, if not central, you opened the file in read only mode (as someone had it open first), but this can't be the case, because the only way to own elements in revit is through worksets = collaboration = central model. SO 99.99999% the scenario explained above is the cause. You, or someone opened the actual central model and now ALL local copies are unable to sync with the file (because it is essentially in read only mode) and all local copies will lose sync. NB. DO NOT OPEN MODELS THROUGH THE BROWSER UNLESS YOU KNOW WHAT YOU ARE DOING...YOU CAN DESTROY AN ENTIRE COLLABORATIVE MODEL THIS WAY!