http error on image upload – again and again…

hi everyone,
i run a blog which functioned fine for quite some time. i created posts, uploaded lots of images, using both the standard wordpress-uploader and the “add from server”-plugin, everything was perfect.
a few days later, i tried to add some new pictures and – http error.
the image can be seen in the library, but if i try to edit it, it says “no image data available” or something like that.
in the meantime, i remembered that i had installed some plugins (statpress reloaded, garmin connect, runners log, nextgen-gallery, youtube-embed) which soon became my prime suspects, of course. i disabled them, i even deleted them – nothing. the user priviledges of the uploads-folder and all subfolders are 755.
guys, please help me. i´m on the ropes right now 🙂
i tried everything i could find on the web. editing the .htaccess-file, using different plugins to upload the pictures. no remedy.
this morning, i thought i´d give it a last try. i deleted the sql database and created a new one. nothing. i deleted the blog from my webspace and installed it again from scratch. nothing. still the http error.
so, what else can i do? leave it this way is no option.
could it be that wp tries to write to a file that has become read-only for reasons beyond my imagination?
or could it be that my hosting company changed some crucial settings? how could i check it out? which settings could that be?
please help, you´re my last hope!