I think the problem is you try to assign "human readable time" into _time while it should be seconds from epoch. Good news is, you already have it from strptime(), ie you don't need those "convert ..." cmds unless you want log_time and log_date for some other purposes.
Maybe this would work better
... | rex field=source "^(?P<path>[^\_]+)\/(?P<log_type>[^\/1]+)\_(?P<ip>[^\_]+)\_(?P<datetime>[^\_]+\_[^\_]+)" | eval _time=strptime(datetime, "%d/%m/%Y_%H:%M:%S") | timechart span=5min count by log_type
If you want use only log_time (ie sum all dates into one) then bucket -cmd could help you. Simply dropping date from above example will make Splunk assume it's current date.
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