Media Uploader Does Not/Incorrectly Processes Uploads

When trying to upload media files (primarily images, usually in .jpg or .png format); we encounter multiple issues.

Using the “modern” uploader (upload.php, main media library page), we receive the error An error occurred in the upload. Please try again later., with no further information given. Upon refreshing the page, a phantom placeholder appears, showing the file name but no preview of the image. Searching the uploads directory via FTP, we can sometimes locate the uploaded file, but more often than not, it is not placed in the correct month/year directory. It never processes the image file into different sizes (presumably for thumbnails and other predefined dimensions). WordPress correctly recognizes and inserts the image in posts and pages, but does not properly show them in the backend.

If we use the fallback uploader (media-new.php), the image is accepted and usually placed in the correct directory, but it is never processed and the webpage does not resolve back to the library (ie after upload, a blank white page is shown).

This behavior has been happening on and off since at least the release of WordPress 4.0. All plugins have been disabled and the default 2015 theme is being used, with the same outcome. Permissions have been verified as 755 for the /wp-content/uploads/ directory. The webhost is 1&1. Any advice/tips given would be much appreciated.