On a previous thread, I was told that the intermittent 500 errors I got on upload and the other queer things that happened in my Media library - including mysteriously disappearing and reappearing thumbnail images, and appearance of the administration sidebar a second time in the Media Library screen - are a hosting problem. I convinced my host that the problem is real by sending them screenprints and error logs, but their level three technicians were unable to duplicate the errors, so they couldn't see whether their attempt to fix it had worked, until I updated the support ticket to tell them that it hadn't.
I'd done EVERYTHING in the support forum troubleshooting guide. As a last resort, I backed up my site, deleted the folder called "wordpress", and uninstalled WordPress in the control panel. Then I installed version 3.4.2 from the control panel, because I didn't recall having had this problem before the upgrade to 3.5. This did not help. The same errors occurred on 3.4.2, so I deleted and uninstalled again, and did a fresh install of 3.5.1, all from the control panel. Note: Doing this through the control panel was more thorough than simply re-installing from the WordPress administration dashboard, as I had already done several times before. I had tried repeatedly and failed to upload WordPress using FileZilla, so this was the closest I could get to a manual upload.
I turned off all comments, deleted the sample page, the sample post, and all the plugins that came with the installation. I still got errors on upload, but my host could not duplicate them.
Looking at the uploaded files in my control panel's File Manager, I see that the files that uploaded without error all had multiple copies in various sizes created by WordPress. (I had read that this is the second stage of an image upload.) The images that got a 500 error only have the original in the uploads folder. This proves that the upload failed in its second stage.
The uploads folder lists file sizes, and I noticed that the slides that got the errors are much bigger than the pictures I'd uploaded successfully. I wanted to see if I could make them smaller, figuring this might help. I did this on the computer where I'd originally created the powerpoint presentation, and saved the pptx file as jpeg, making a new folder of slides. Each of the new slides was less than a tenth of the KB of the corresponding old slides.
The newer, smaller slides uploaded without error. Even the largest of the first set of slides was only 1007 KB, well within the 20 MB (=20,000 KB) limit for file upload, so the size alone didn't seem to be an adequate explanation for the failure of the larger ones to upload. I then tried and succeeded uploading a much bigger (4 MB = 4000 KB) jpeg photo without error, which I saw in file manager has been reproduced in various sizes, proof that the upload of that file completed its second stage, and that size is not the reason why these slides didn't upload.
Visually, the first set of slides looks perfect, but evidently they were badly corrupted by being produced in a computer that didn't support powerpoint editing, badly enough to prevent WordPress from making copies of them in various sizes.
While thirteen evidently corrupted slides sat in my uploads folder (in their original form only), my Media Library had some really weird effects. The WordPress administration sidebar sometimes appeared a second time in the middle of the Media Library screen. The thumbnail images of these slides, as well as of all the other pictures I'd uploaded, would mysteriously disappear, some or all of them at a time, reappearing days or hours later, for no apparent reason. While a file's thumbnail was missing, it was impossible to view it in the edit mode, to attach it to a page or post, or to see it in a page or post. Although upload of the corrupted slides almost always failed (and once even crashed my website), other files sometimes failed as well, making the cause of the problem very hard to diagnose.
Could all of these weird effects be caused by the evidently corrupted slides sitting in the upload folder, or is this truly a hosting problem? My host is clueless what caused these errors. My contract with them is about to expire. I have backed up my site to my local computer to do all this testing, and I'm going to have to rebuild it from scratch anyway. Please let me know if I'd be better off switching hosts, or if all of this is my fault for trying to save a powerpoint presentation as a folder of jpeg slides, using a computer that doesn't support powerpoint editing.