Can't buy any prestige upgrades. I have the points, but when trying to buy even the first upgrade the box just closes when trying to click on the purchase button.

Sorry about that. Are you using ultra-wide monitor? There were similar reports, and changing resolution to a regular 1920x1080 seemd to help.

I have another issue purchasing upgrades.

I have exactly 2 prestige points and I'm not allowed to purchase an upgrade that costs "2.00".

Maybe a rounding issue?