Resolution:
When "rotatelog" is run in tibemsadmin to rotate the log manually, "Error: Unable to rotate the log file" is shown in tibemsadmin. In the EMS server log, there is also an error. Example, "ERROR: Unable to rotate log file 'ems.log' as 'ems.log.3'. Automatic log file rotation disabled."

This could happen when the log file already exists in the log directory. In this case, all files with the name should be removed.  Command "rotatelog" will work again. When EMS is started, it queries the log directory to find the maximum number appended to a log file name (i.e ems.log.*).  Each time when EMS tries to rotate the log file, it increases the last log file number by 1 and tries to create the file. If it happens that that file already exists (i.e., moved to the log directory after EMS is started), the log file can not be rotated.

For other issues regarding log rotation, refer to KB 29487.

Error: Unable to rotate log file.

KB 29487