I am unable to duplicate your issue on at least 3 different servers and hosts, and two different types of operating systems.
You need to contact your host. We have no control over why your host has put in those limits to your account. There is nothing within UBB.threads that would limit your images in landscape but not in portrait for post attachments.
Be aware that whatever image processing tool you used to rotate your images, they could be stored with new compression, at a smaller file size. Your original image could be too large for your current web hosting account's setting, and your manipulated image would be smaller. This all depends on how you are rotating -- via pixels or EXIF data
Also as stated above, attempt to use GD in place of ImageMagick. The toggle to do that can be found at:
Control Panel > Attachments > Graphics Library To Use For Image Processing