To avoid any doubt of overlapping or incorrect conditions I have created this scenario in a new installation of Joomla localhost

I created a field (radio button), named "cb_scelta" with two options.
Option 1 = "Ridotto"
Option 2 = "Completo"

Then I created a field (text) conditional upon "cb_scelta" field.
if "cb_scelta" = "Completo" then show

In the registration and in this situation the field is displayed correctly only if "cb_scelta" = "Completo." Otherwise the field is not displayed.
The problem comes when I confirm the registration and then save. The value entered in the text field while recording is not saved in DB.

If, instead, the scenario change of the condition, and then

if "cb_scelta" [Not =] "Ridotto" then show

In the registration and in this situation the field is always displayed. I can hide it if I choose "Ridotto". Instead I display the field if I choose "Complete".
In this case when I confirm the registration and then save the data the value entered in the text field is saved correctly in the DB.

I hope I have explained well, please try to confirm the scenario for the bug and can find a solution.
Thanks for the support

The error may depend on PHP version I use?
I'm using version 5.4.27.
Joomla 3.6.4
CB 2.0.15+build.2016.11.17.23.30.51.00b5b220a
CB Conditional 4.0.7+build.2016.10.28.01.27.57.1750676ea

Hi krileon, the problem is not whether the field or Tab is displayed when conditioning but saving the conditioned field data.
How did you write here the problem arises when the field that generates the condition is set as read-only.
I checked, and if the field is removed the check read only then it works and the conditioned field value is saved correctly.
We look forward to the resolution of the problem, thanks for the support