Reproducer:
postgres=# CREATE TABLE t AS SELECT ''a FROM generate_series(1,99); CREATE TABLE u AS SELECT ''a FROM generate_series(1,99) ; VACUUM ANALYZE t,u;
postgres=# explain SELECT * FROM t JOIN u ON t.a!=u.a;
ERROR: could not determine which collation to use for string comparison
HINT: Use the COLLATE clause to set the collation explicitly.