I am experiencing the same issue except there are a few differences. In my case the config file trying to be displayed is greater than 4M. All the other devices I am collecting configs for are collecting fine but their sizes are 29K or less. Also, the banner I see is different since this device was discovered using ./snmp-scan -P. If I try to delete this device LibreNMS fails to remove it from the system. Below is the output I am see when trying to navigate from the Availability map.
Juniper MX480 JD_router
System Name
JD_router
IP Address
10.13.0.5
Hardware
Juniper MX480 Midplane Internet Backbone Router
Operating System
Juniper JunOS 12.3R6.6
Serial
JN12FE345ABC
Object ID
.1.3.6.1.4.1.2636.1.1.1.2.25
Contact
John Doe
Device Added
5 days 16 hours 14 minutes 28 seconds ago
Last Discovered
2 hours 23 minutes 30 seconds ago
Uptime
154 days 21 hours 36 minutes 47 seconds
Location
ABC C46
Lat / Lng
N/A
View
Overall Traffic