I can't find the "50" as part of the "Unable to print" error. Usually when you get "Unable to print", it means there's a torn piece of paper, paperclip, some foreign object in the machine somewhere. Check under the scanner lid and inside the machine, look thoroughly; you may find a piece of paper blocking a sensor.
If the error message continues, follow these steps before disconnecting the MFC so
you won't lose any important messages.
Then, disconnect the MFC from the power
source for several minutes (which will erase all faxes), and then reconnect it.
Check whether the MFC has faxes in memory:
1. Press Menu/Set, 9, 0, 1.
2. If the MFC has no faxes, disconnect the MFC
from the power source for several minutes
and then reconnect it.
If faxes are present, you can transfer them to
another fax machine. Go to Step 3.
3. Enter the fax number to which faxes will be
4. Press Black Start.
5. After faxes are transferred, disconnect the
MFC from the power source for several
minutes and then reconnect it.
You can transfer the Fax Journal report to
another fax machine by pressing Menu/Set, 9,
0, 2 in Step 1.
Let me know if it helps!