Fixed: IIS request filter and PHP upload limit was set above 2GB on a 32bit system. When dealing with the overflow, php dumps the result as a negative value on some instances but not on others (the bits that worked). Reset all the hard limits and added $CFG->maxbytes = 1610612736; (1.5GB) to the config.php file.
General developer forum
Broken image uploads (2.6.1)
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