As for the <nil> errors, @allejuppa already created a PR (and it was merged) to give the correct error information.
I can't be of too much help, except to suggest that most of the network is experiencing a similar number of errors in their logging. I think at this point we've addressed some of these errors, but we're continuing to work on the renter and host protocol right now.
The password that it's asking for is your current wallet password. The 0.3.3.1 wallet did not have a password. This should be more clearly specified by the frontend. Sorry for the confusion.
The reason it needs your current password is so it can encrypt the wallet you are importing. It's not a good idea to leave unencrypted keys on your disk, but you can't encrypt things unless you've got the password.
@in-cred-u-lous I'd rather release something that acknowledges the actual limits of Sia than something that's going to have a user uploading >100 files and run into a bunch of unexpected problems. Obviously it's a fix until the scalability solutions are in place. If we could get the scalability stuff finished in 1 day we would release it but there's a lot of code that needs to be written and tested to make that happen. We ARE dealing with the problem head-on.
Further, it's not the network that's being crippled by all of the files, it's your local machine. siad starts to struggle at 100 files because the process that the uploader uses to scan and upload files is intolerant of large file volumes. A rogue uploader isn't the problem, it's your local state.
We do plan to add synchronization tools at some point. Currently, there's no API for editing files, which means you'd have to delete-and-reupload to change a file. We already have a protocol written that will safely enable making modifications to files without incurring huge upload overhead, but will take some time to get to that.
John, no worries, you haven't lost them at all. I thought the same thing when I did that upgrade, but what it turns out to be is that your 5.1 wallet takes a bit of awhile to sync. Once it finishes doing that all your siacoins will magically reappear :). I know because it happened to me too. See the current block here: http://explore.sia.tech/
To restore your files you should copy renter.json, hostdb.json and all other files that end with ".sia" from the renter/ directory in your old sia installation to the new one. Then your files should show up in the client again.
Hard to diagnose without the full error message -- the next release should log these error messages in their entirety somewhere. My best guess is that the Rename operation is failing when the .sia files is saved. Check your renter folder for .sia files.