I have a schema in which I have a table ALIGNMENT and a snapshot log:

CREATE MATERIALIZED VIEW LOG ON ALIGNMENT WITH PRIMARY KEY;

Materialized view log created.

Then, in another schema, I try to create the snapshot:

CREATE MATERIALIZED VIEW ALIGNMENT BUILD IMMEDIATE REFRESH FAST AS SELECT * FROM ALIGNMENT@SYNCH;

and it hangs (I left it overnight). So, I guess this points to a lock issue, but I didn't see anything in DBA_LOCK referencing either of my schemas. Any idea what I should look at next?

I'd start with looking at the alertlogs (but at the site your building and the originating site)

Good luck!