Definitely odd behaviour, but not the "cannot select cells with my cursor" claimed in your initial post.
In the GIF supplied, after several clicks on cells returning no response, you clicked on cell D3, then Cell C3, with this result:

Each of those clicks resulted in the clicked cell's address being inserted into the formula being built. Further clicks also added to the formula.
Later:

Here the SUM function was likely entered from the keyboard, and the two range references may have been entered that way as well.
Not an issue I've seen before, so not much help from this corner. Thanks for the example. Badunit seems to have a better handle on the issue, and, I hope, may reach a solution.
Regards,
Barry