This is the reason:
https://issuetracker.springsource.com/browse/STS-3739
Springloaded jar was moved and renamed in Grails 2.3.7. GGTS 3.4.0 (or earlier) can't find it.
This bug is fixed in 3.5.0.
These are your options:
1) keep using older version of Grails (<=2.3.6)
2) Download and use GGTS 3.5.1 release.
This information came from here: http://forum.spring.io/forum/spring-projects/springsource-tool-suite/745972-sts-3-4-0-and-grails-2-3-7-run-app-broken?p=746042#post746042
I just updated it a little to reflect that 3.5.1 release is now available.