I'm experiencing an issue with an update to a MSSQL Server 2012 database from PDI 5.4 CE

The error returned is:
Code:
2016/11/30 17:01:58 - Update.0 - ERROR (version 5.4.0.1-130, build 1 from 2015-06-14_12-34-55 by buildguy) : Error in step, asking everyone to stop because of:
2016/11/30 17:01:58 - Update.0 - ERROR (version 5.4.0.1-130, build 1 from 2015-06-14_12-34-55 by buildguy) : org.pentaho.di.core.exception.KettleDatabaseException: 
2016/11/30 17:01:58 - Update.0 - Error inserting/updating row
2016/11/30 17:01:58 - Update.0 - The UPDATE permission was denied on the object 'Task', database 'WTS', schema 'dbo'.
2016/11/30 17:01:58 - Update.0 - 
2016/11/30 17:01:58 - Update.0 -     at org.pentaho.di.core.database.Database.insertRow(Database.java:1280)
2016/11/30 17:01:58 - Update.0 -     at org.pentaho.di.trans.steps.update.Update.lookupValues(Update.java:189)
2016/11/30 17:01:58 - Update.0 -     at org.pentaho.di.trans.steps.update.Update.processRow(Update.java:318)
2016/11/30 17:01:58 - Update.0 -     at org.pentaho.di.trans.step.RunThread.run(RunThread.java:62)
2016/11/30 17:01:58 - Update.0 -     at java.lang.Thread.run(Unknown Source)
2016/11/30 17:01:58 - Update.0 - Caused by: java.sql.SQLException: The UPDATE permission was denied on the object 'Task', database 'WTS', schema 'dbo'.
2016/11/30 17:01:58 - Update.0 -     at net.sourceforge.jtds.jdbc.SQLDiagnostic.addDiagnostic(SQLDiagnostic.java:368)
2016/11/30 17:01:58 - Update.0 -     at net.sourceforge.jtds.jdbc.TdsCore.tdsErrorToken(TdsCore.java:2820)
2016/11/30 17:01:58 - Update.0 -     at net.sourceforge.jtds.jdbc.TdsCore.nextToken(TdsCore.java:2258)
2016/11/30 17:01:58 - Update.0 -     at net.sourceforge.jtds.jdbc.TdsCore.getMoreResults(TdsCore.java:632)
2016/11/30 17:01:58 - Update.0 -     at net.sourceforge.jtds.jdbc.JtdsStatement.processResults(JtdsStatement.java:584)
2016/11/30 17:01:58 - Update.0 -     at net.sourceforge.jtds.jdbc.JtdsStatement.executeSQL(JtdsStatement.java:546)
2016/11/30 17:01:58 - Update.0 -     at net.sourceforge.jtds.jdbc.JtdsPreparedStatement.executeUpdate(JtdsPreparedStatement.java:504)
2016/11/30 17:01:58 - Update.0 -     at org.pentaho.di.core.database.Database.insertRow(Database.java:1247)
2016/11/30 17:01:58 - Update.0 -     ... 4 more
The error suggest that this is an issue with permissions of my user connecting to the database...
Database connection is ok (test) and a select using the connection works as expected.

The strange thing is that when running the same transformation, from the same database repository on the same Windows 10 system, but using PDI 6.1 CE instead of PDI 5.4 CE, the transformation runs without issue.

My first assumption was that this must be an issue with the driver. I have checked the driver in the \data-integration\lib folder of both environments, and they have the same driver (jtds-1.2.5.jar).

I've tried to copy the jtds-1.2.5jar file from my 6.1 environment to 5.4, without success.
I've also tried switching to the database connection to use the native driver, and copied the sqljdbc42.jar file to the \data-integration\lib folder, but the error message remains, which seems to suggest that the native driver is not being used.

This makes no sense to me, so any insight or advice is highly appreciated.

Thanks!