badMessage / Illegal character warnings during upgrade #4017

Today I did an upgrade from 8.0.16 to 8.0.27. I naturally assumed it would go smoothly, because QB is usually just that awesome. But, instead I got this (at the start of the upgrade process):

2018-12-03 13:24:42,462 INFO  - >>> 2018-12-03 13:24:42,461 INFO  - Booting QuickBuild from '/opt/qbserver-8810'...
2018-12-03 13:24:42,463 INFO  - >>> 2018-12-03 13:24:42,462 INFO  - Starting framework...
2018-12-03 13:24:42,487 INFO  - >>> 2018-12-03 13:24:42,487 INFO  - Caching plugin libraries...
2018-12-03 13:24:43,620 INFO  - >>> 2018-12-03 13:24:43,620 INFO  - Starting QuickBuild server...
2018-12-03 13:24:43,781 INFO  - >>> 2018-12-03 13:24:43,779 WARN  - Illegal character 0x16 in state=START for buffer HeapByteBuffer@7bb26d03[p=1,l=244,c=8192,r=243]={\x16<<<\x03\x03\x00\xEf\x01\x00 etc... etc... 0\x00\x00\x00\x00\x00\x00\x00\x00}
2018-12-03 13:24:43,781 INFO  - >>> 2018-12-03 13:24:43,779 WARN  - badMessage: 400 Illegal character 0x16 for HttpChannelOverHttp@68c63fa4{r=0,c=false,a=IDLE,uri=}
2018-12-03 13:24:43,977 INFO  - >>> 2018-12-03 13:24:43,976 INFO  - QuickBuild version table found. Catalog: QUICKBUILD, Schema: PUBLIC, Name: QB_SETTING, Type: TABLE
2018-12-03 13:24:44,004 INFO  - >>> 2018-12-03 13:24:44,004 INFO  - Stopping QuickBuild server...

The upgrade then went on and claims to have completed successfully. But of course I am worried about data corruption.

Can you tell me what happened and what I should do to mitigate?