There could many reasons why a HTTP 503 error is occurring:
1. Check to make sure if an external database is used, it is up and running.
2. What does your registry.log say? can you post some of the errors here?
3. If this the first time you are seeing this error, or has this worked before and then all of a sudden stopped working?
You ought to upgrade to a supported release since LISA 7.5 is EOL:
DevTest Solutions Release and Support Lifecycle Dates-CA Technologies