Hi Folks,
I had exactly the same issue…fortunately I’ve only got 13 blogs so I was able to fix it manually.
For anyone that is interested, this is what I did. I’m no mySQL expert so this is very manual (I tried to upload the records using ‘import’ but as usual some setting was correct…couldn’t be bothered dickin’ with it so I went manual 
)
It seems that the BP 1.2.4 empties the following Db Table – ‘wp_bp_user_blogs’.
Fortunately I did a backup before venturing with yet another notorious BP Update.
In my Db Backup (which is pretty big), I just did a ‘Find next’ search for ‘user_blog’. The info. your looking for looks like this:
“LOCK TABLES wp_bp_user_blogs WRITE;
/*!40000 ALTER TABLE wp_bp_user_blogs DISABLE KEYS */;
INSERT INTO wp_bp_user_blogs (id, user_id, blog_id) VALUES (1,1,1),(2,3,1),(21,1,14),(20,1,13),(19,6,12),(17,1,12),(22,1,15),(23,1,16),(28,1,18),(25,1,17),(26,10,16),(27,10,13),(29,1,19);
/*!40000 ALTER TABLE wp_bp_user_blogs ENABLE KEYS */;
UNLOCK TABLES;”
If you open you Db in myPHP Admin, click on the ‘wp_bp_user-blogs’ table, and when it’s empty row appears, just go up the top and click ‘Insert’. This will present you with a list of the 3 rows in the table. Their ‘Field’ names are respectively ‘id’, ‘user_id’ and ‘blog_id’ as it says above.
Just take each block of 3 numbers in from your backup info. (I’d advise not using my numbers above ), and pop them in, to the respective rows, 1 set at time. That is to say, place 1 each of the first set of 3 numbers on each row in its respective Field, then click ‘Go’. That will create a row in the Table. Then repeat the procedure for each set of 3 numbers.
I hope that helps someone.
Next MAJOR Problem is that I no longer have BP Admin Bars appearing at the top of my Blogs. If anyone has a fix I’m all ears.
These are prettty major issues. This is my first BP Update…I’m now terrified of future ones.
Anyway, now I’m off to spend more hours troubleshooting this other MAJOR PROBLEM.
Cheers
Stephen G
