You right, I wrote about not newest version. I have used version 2.3.2, because a website is old too and based on Joostina that uses Joomla 1.0 engine, so version 3.x is not applyable.
When I moved default page from frontpage I have been surprised and after simple googling I go here.
I'm understand that last posts was so old, but, IMHO, there is some people who still uses old version and probably can see that problem.
When I found solutions for any problems I trying to share it, and, I think, support forum is a best place for it! :)

Hi!
Yesterday I encountered a similar problem.
Default page was changed from com_frontpage to blank static content page.
When JoomSEF turned off everything worked fine, but if I turn it on and type in address line something like domain.tld/ I see contents of com_frontpage.
After reading this topic (and some other forums), and found no solutions, I have analysing sources of JoomSEF, and find a solution in 10 minutes.
So I'm very surprised that the tech support didn't find it so long, or maybe find, but for $$$, I dont know that.
Finally, exactly solution:
In /components/com_sef/joomsef.php, approximately on 70 line, find:

and replace it to:

I apologize to the developers for changes in the code, but it really works!