Hi experienced_tester,
Sounds like you are coming across the limitation that MT4 has with FXT files being larger than 2GB..
These posts should assist you:
viewtopic.php?f=4&t=176
viewtopic.php?f=4&t=202
The reason why you could test longer periods before is because you were not using tick data which obviously is much more voluminous.
Hope this helps.