I solved the sync issue following info here:
https://youtrack.jetbrains.com/issue/SCL-13024
Key info is “It seems that sbt is trying to re-resolve ‘sources’ and ‘docs’ for sbt plugins that are not available at all. If properties to download “Library sources” and “sbt sources” are disabled, then time to dump the structure is reduced from ‘forever’ to less than 1 minute.”
That is , in IntelliJ IDEA, clear the check box “Library sources” under “Project level settings” for Build Tools sbt in IDEA preferences. As shown in image below:
