Ran into this exact same issue. Uninstalled eclipse/my jdk/even tried things like deleting .minecraft and reinstalling forge.
What I noticed is that whenever I ran ./gradlew genEclipseRuns it was building ~very~ fast, so I downloaded another version of the sourceForge mdk and rebuilt and it worked.
I went with mdk 31.1.30, because my second guess was that this might be a new uncaught bug.
But my guess is that it just has to be a different version, so maybe 31.1.48 or something.
My guess is there is something being cached by gradle for that initial eclipse build from the newer java version.
Maybe a faster way to fix this can be done by looking into ...\.gradle\caches and deleting forge related caches.
Cant open minecraft with Eclipse
in Modder Support
Posted
Ran into this exact same issue. Uninstalled eclipse/my jdk/even tried things like deleting .minecraft and reinstalling forge.
What I noticed is that whenever I ran ./gradlew genEclipseRuns it was building ~very~ fast, so I downloaded another version of the sourceForge mdk and rebuilt and it worked.
I went with mdk 31.1.30, because my second guess was that this might be a new uncaught bug.
But my guess is that it just has to be a different version, so maybe 31.1.48 or something.
My guess is there is something being cached by gradle for that initial eclipse build from the newer java version.
Maybe a faster way to fix this can be done by looking into ...\.gradle\caches and deleting forge related caches.