UPDATE: turns out that php 5.2.x was causing this. an upgrade to 5.3.3 fixed the issue.

Once i have created a UDF page which contains 11 fields [in my case 10 textfields and an email field] and publish it, attempting to add another [or any amount] of fields fails, and results in an endless "saving..." message and in nothing getting saved.
This includes modifying fields, or any other tab in the CMS, even stuff that is not UDF relevant.
If i click on delete for the 12th element all is normal again.

Using 2.4.3 and trunk userforms.
The POST response is completeley empty, yet the request returns a 200 code. No javascript errors are present.

All logs and all debugging measures i know of or i have been suggested to try on IRC have been attempted, but no information or no error at all [at least not regarding this] shows up [but does if i say, ave a syntax error somewhere, so logging IS working.]

I can even not delete the page from the CMS and have to go through manually deleting the page from the DB or by using "bulk actions".
Any help on how to solve this is appreciated.

Please advise.
Other users have attempted to reproduce, but without success.