Solved it myself if anyone is interested, though I'm still a bit confused...

When making it live I disabled the marketplace with the following line in the config file so the client couldn't mess up the site by downloading a new theme.



In /concrete/elements/block_area_add_new.php (line 173) I've just found an if statement that only shows the contents of the Add Block dialog if the marketplace is enabled.



Surely this is a bug, because disabling the marketplace shouldn't stop you from adding a block to a page.

I am having a very similar issue. I moved my C5 (version 5.5.2) site from subdomain to the primary domain on the same host server. I did not recreate the database, only moved the site files. Everything seems to work fine -- I can install new blocks and edit existing blocks on a page...but when I try to add a block, add a stack, or add anything from the clipboard, I get a blank "Add Block/Stack/Clipboard" dialog.

I made sure only my new URL was associated under my c5 project page (at first, both my subdomain site and primary domain site were included under one c5 project). I am connected to the project community. I tried removing the if statement described above from the /concrete/elements/block_area_add_new.php with no effect. I also tried including a BASE_URL and DIR_REL statement in my site.php even though 5.5.2 doesn't need it.

I don't know what else to do. I may need to totally start from scratch and install a new site. I would appreciate any help I can get.

Did you upgrade to 5.5.2 from your root/upgrade/ folder or was it a fresh install of 5.5.2?

It was a fresh install -- I built the site on the subdomain about a week ago and just moved the site files to the root folder last night.

I should also add that at one point I had both the subdomain site and the primary domain site up (pointing to the same database) and i was still able to add blocks just fine on the subdomain site (and the change would show in the primary domain site. Likewise, changes I made to existing blocks in the primary domain site would show on the subdomain site.

I stupidly deleted the site files from the subdomain folder thinking that would help -- the primary domain site was unaffected by deleting the files, but now I have no backup to add new blocks. I am currently restoring the site files to the subdomain site to see if i can still add a new block there at least.

After working with the edit_area_popup.php file, I discovered that for some reason my admin account does not have the 'canViewPane' permission on the primary domain site, but apparently does on the subdomain site. I am using the original admin account in both sites (shared database, so same account permission should apply to either?). I've tried editing my permissions in the primary domain account but that doesn't work.

Concrete5.5.2.1 has just been released,
In your position, I would cut my losses,
Delete the database tables, delete the existing installations and start from scratch with a fresh install of concrete5.5.2.1 onto a clean database,
I know this is not want you want to hear, but unless your installation is sound, you may continue to unearth more problems in the future,
Best Regards
Weyboat

OK, I have no idea what I did, but this randomly starting working fine even though I hadn't changed anything within the last few minutes. I don't know if it just took that long to reconcile the connection to the marketplace or what...it makes me very nervous to not know why it's fixed. I don't know whether I should take it or do a fresh install (I guess I know the smart-but-not-fun answer to that question).

I appreciate your speedy response in any case!