This is very likely to be due to PR1241326. This problem affects 15.1Rx and will be resolved starting in 15.1R6. Only EX2200 and EX3300 are affected. 12.3Rx is not affected.
In 15.1Rx, the workaround is to play with the number of bytes in the Radius packet to change to an even number.
https://prsearch.juniper.net/InfoCenter/index?page=prcontent&id=PR1241326