Firstly, what file manager are you using? It looks like MonstaFTP, but we removed it because it was too buggy. So if it doesn’t work, then that’s your answer: the file manager is buggy.
But it also tells me that you’re probably using a third party file manager. Please know that we strongly recommend against ever entering your credentials into third party websites, there is no way to know what they’ll do with them.
I’m pretty sure that RoundCube doesn’t work on our hosting. We had to disable IMAP functions on our hosting due to rampant abuse (the majority of accounts using it were using it to try and break into other people’s mailboxes).
As for the log_errors issue, that’s a very weird issue from RoundCube to have. We don’t have it enabled because error logging was too costly. But for RoundCube to completely refuse to function because of that is a very strange design decision. It’s up to the server and/or website administrator to decide how errors are handled, not the application developer.