Migration problems

HI,

We've migrated our Drupal 7 site from one host to another ISP. Now the site has several issues and I am too much of a newbie to figure it out. I suspect a lot has to do with the rewrite rules and directory permissions, but don't know how to fix them. Here are the problems with which I would appreciate any help:

1. The CSS files and images are not loading. E.g. according to the source of my webpage it is calling many CSS files. Just to pick an example the first one is like this:
http://www.example.com/modules/system/system.base.css?lf77qa (I have my domain instead of the example.com part, of course.)
If I look at the directories via SSH/FTP I see that the system.base.css is in the folder where it should be (/public_html/modules/system/system.base.css ) So why isn't it loading ? (If I got to the URL http://www.example.com/modules/system/system.base.css I get a 404) And what does the "lf77qa" part do at the end of the URL?

The same applies to my images they are there in the directory structure where they should be but they are not loading; e.g. http://www.example.com/sites/default/themes/mythemes/acq_sky3/screenshot...

2. I get a lot of error messages like this on every page:
The specified file temporary://filew3FJZ3 could not be copied, because the destination directory is not properly configured. This may be caused by a problem with file or directory permissions. More information is available in the system log.
But the error log is not really helpful. E.g. I found only this there

Location http://www.example.com/misc/favicon.ico
Message File temporary://filew3FJZ3 could not be copied, because the destination directory is not configured correctly.

So what directory's permission should I set to what in order to avoid these kind of errors?

Thanks for any pointers in advance