Directly after a unsuccessfully full backup could you do:
grep "crypt_openssl.c.o" /usr/local/var/urbackup/data/filelist.ub
Would be great if it is a file with distinctive enough file name that it only occurs once in your backups.
Otherwise perhaps there are some more hints in the debug level logs files. See Having problems with UrBackup? Please read before posting on how to create them.