Note: This issue usually appears in older (Pre-9.3) versions of JSCAPE MFT Server
A "java.io.IOException: java.lang.ArrayIndexOutOfBoundsException: 0" usually means you have a corrupted account datastore file. An account datastore file is either a set of .usr files, if using "file record" datastore or a single accounts.dat file, if using "file" datastore.
The most telling sign of a corrupted account datastore file is that it is zero byte is size. Check for zero byte sized .usr files inside the account datastore directory and restore them from a known good backup or delete them and then manually recreate them using Server Manager if you have no backup. If you are using "file" datastore then restore the accounts.dat file.
Please follow these steps:
1) Stop JSCAPE MFT Server
2) Find the zero byte sized .usr files or the accounts.dat file found inside the account datastore directory. This directory is by default located in <mftserver installdir>/users/<domain> but it is customisable so please find the appropriate directory if you have customised it in the Datastore module (see screenshot below)
3) Remove the zero byte sized .usr files and restore the appropriate backup if you have any, if not then you will have to recreate them manually using Server Manager. If using "file" datastore then restore the accounts.dat file.
4) Start JSCAPE MFT Server