As noted in the documentation the ExternalFreeze() can not last longer then 10 minutes or however long it takes you to run out of global buffers.
You could use irisstat :
iris stat [INSTNACE] | grep WDSUSPD # this will show you if the write daemon are suspended.
iris stat [INSTNACE] -W #this will resume them.
As a guess you are running out of buffers and the options are to move the snapshot to a less busy time on the system and/or increase the number of global buffers.
Look at using mgstat to find a better time to do the backup.
Is it possible to define aliases per IRIS instance?
Looking at Aliases mentions the .iris_init but that is user wide.
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iris session [INSTANCE] -U %sys ALL^%SS would give you the PID,namespace, routine, CPU and globals.