Fixed problem re tempnam(): in attempting upgrade from Drupal 7

Update: I started writing this about issues I had; did a bit more research, tinkering, while writing, and seem to have fixed these issues. But putting it into the thread here in case someone has the same/similar issues, may be helpful.
[Admin: so, welcome to delete it if it's just cluttering up the forum.]

I hope to upgrade from multisite Drupal 7 to backdrop.

Created a backdrop folder, to put Backdrop, sites etc into; see if can get this working, while leaving the Drupal files untouched for time being.

Tried with one, smaller site. Hopefully got folders etc about right: a www.example.com folder within sites, for instance.
At first, I got error message about the files/config_d41d8cd98f00b204e9800998ecf8427e/active folder being empty.
Found some info here about similar errors. I edited the database password, to remove a character that's not a letter or number, as saw this might be an issue. Still got the error message. But, decided to try update.php; and this indeed performed updates, and moved me beyond the error message - so could see site content, do some administration.

But then, pages had error messages like this:

• Notice: tempnam(): file created in the system's temporary directory in backdrop_tempnam() (line 2623 of /usr/www/users/hkout/backdrop/core/includes/file.inc).
• Warning: file_put_contents(temporary://filepAQffF): failed to open stream: "BackdropTemporaryStreamWrapper::stream_open" call failed in file_unmanaged_save_data() (line 2078 of /usr/www/users/hkout/backdrop/core/includes/file.inc).
• The file could not be created.
  
  I suspected there's a need for a tmp or temporary folder somewhere, with permissions to create files etc within it; maybe this is lacking. Did find it.
  
  - and having changed permissions for tmp since writing this, I had less errors, like this:
  
  The specified file temporary://filepaJeIP could not be moved/copied because the destination directory is not properly configured.
  
  The Backdrop log notes two main errors, it seems to me:
  The file permissions could not be set on public://js.
  The file permissions could not be set on public://css.
  
  Googled, and found info about Drupal; refers to js and css folders within "files". I found these, changed permissions; and the errors went away, leaving the site apparently looking pretty good.
  
  - It's simpler than the other sites, so expecting more hassles with these, partly as there are no longer modules such as Gmap, Node Gallery. But right now, feel quite pleased to have got this far!