So now it has been 7 months and no feedback, nor solved in 5.0.
Will try and ask again -> has this been reported yet?
Still occurs, tested on two different computers, one with AMD and one with Nvidia GPU's (SLI).
Edit: tested on W7 + W10
I personally haven't gotten this issue anymore for quite a while and my screenshot directory is rather big. Did you modify an files in the client? It could be as simple as enabling the chat log or messing around with graphic settings.
I know there's a specific setting that can mess up creating screenshots and cause a massive freeze, but you can only do this by messing around with the system.cfg, which you shouldn't do.