I've also had this issue, but one step farther. We have created collector applications using custom PNG symbols, and we get the 'Prohibited' symbol, but only on some of the field tablets (as in same make/model but about half are fine and the others are experiencing trouble). It works fine on iPhones, and all tablets can use the customized symbols just fine in the Explorer for ArcGIS app on their field equipment. Very strange! I hope someone figures out the answer! I love being able to use custom symbol!
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