As a hobby I restore cherbini hunters, and Alberg 30s have done 4 so far and on my fifth, Pretty much restore them to 98% new condition. I have run into this on all but one hunter so far, These rudder cracks on one side of the rudder, or mostly one side.
One thing to consider that I have found with all 4 rudder repairs I've done is that when they build the rudders they often didn't center the blade stock (runs horizontal to the rudder post) on the inside in the middle of the rudder. They are often offset on direction or the other. why? dont know! seen it quite a few times. Also one of my feelings is that in the cherib 27 i've restored that have wheel steering, Often the Owners forget to make sure the cap on the emergency tiller is on...rain and spray will get into this spot and build up water in the rudder as the post runs unsealed right into the foam. Also often the seal from were the SS post enters the rudder stock starts leaking and will suck in water over time. The frame in the rudder is usually some SS stock of about 2" by 3/16 thick running horizontal to the SS rudder post. now when it freezes the wet foam between the flat rudder stock and the outside of the rudder causes horizontal cracks along the stock, usually on one side(the side the rudder is leaning inside towards. So often you get 2-4-6 horizontal cracks because there is usually 3-4 horizontal ribs to the ss post. Not saying this is what it is... but worth checking out.. I am sure you drilled some holes to check for dirty foam? should be white or off white foam,, if its yellow then its probably been wet... Hopefully you drilled some holes at the ends of each crack in the fiberglass to stop the travel of the crack. Just a thought.. make sure if you have emergency tiller to seal the cap and to check the area round the SS post and the rudder attachment and seal it up good with some 5200. In time this will have to be addressed to find the cause.. If and when you do, best course is to use a multi tool or grinder and cut out the fiberglass 2" from all edges and carefully peel it off and check out your rudder interior on the side of the most cracks. careful to peel off carefully so you can use the skin to re apply over the repairs.
On two of the 4 rudder repairs i did I found the the fiberglass skin thickness always varied from very thin to very thick and in between. If and when you repair the rudder feel free to wrap the whole rudder in another 1-3 layers of biaxial mat if its thin.
Just information not a verdict of what your actual problem is, but sadly it more appears that has probably happened.
Here is a picture of how the rudder in 1982 27 is designed inside.. very poor I think support inside.
if you look carefully you can see that the upper horizontal stock is close to the outside edge more than the other.. So if your rudder has water in it, it can develop the horizontal cracks your rudder is show due to the expansion of the horizontal stock. In all of the rudder repairs ive done on my boats or friends so far... they all sounded solid didnt have the obvious sound of delamination.
Good luck.. just more info to evaluate your problems