TYPO3 Core

Is not present in 7.6.9
Is present in 7.6.10

If an input in TCA has the option 'eval' => 'null' (or a variation of null, trim, double2,...) the field AND the checkbox for disabled field are disabled in typo3 backend.
This results in a state where disabled fields (null-fields) can not be enabled again since the enabeling checkbox gets disabled with the field and is therefor not clickable.

• changed back to 7.6.9 to verify that it is the core.
• manually (in browser) removed the "disabled" class form the surrounding div.t3js-formengine-field-item - which made the field work again.
• diffed the backend-Extension 7.6.9 and 7.6.10 and appended it to this issue

I hope I assigned this correctly. If you need more Information please let me know what you need.
Best Regards