I opened Software and Updates (software-properties-gtk) from Synaptic Package Manager and when I closed it Synaptic Package Manager kept stucked that way , everything grayed, and placing the cursor on the window shows the loading animation but even after leaving it opened for over 1 hour it never unblocked itself.
When I checked its memory usage I saw it was consuming very few, usually when a software stops responding is using a growing amount of memory or has reached the maximum available, till I have to force-close or the system asks me if wait or terminate. I couldn't close it normally, doesn't work in this cases, it's not the first time that certain windows stop responding but usually it's temporary, not permanent.
I also had an issue with Synaptic PM yesterday. It failed to start, showing a dpkg error and proposing a fix. I tried uninstalling (to then reinstall) but it would not uninstall, even after a boot. I then entered the suggested terminal command fix for the dpkg error (to do with certificates) and then Synaptic PM opened OK. I have not tried to uninstall since, but was unusual behaviour from a trusted app.