gdb drops connection (invalid hex digit 120)

I can't get pyocd gdbserver to work. When I try to connect with arm-none-eabi-gdb, gdb says Reply contains invalid hex digit 120 and drops the connection.
Since debugging the same application with openocd works, I assume the problem is somewhere in pyocd.
The strange thing is that gdb complains about the invalid hex digit when pyocd sends b'$Text=0;Data=0;Bss=0#04' which is a constant so it's not clear to see where this invalid value comes from.

pyocd version is the current dev version (5a52ccc). Full pyocd log is attached.
pyocd.log

$ arm-none-eabi-gdb path/to/application.elf

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Reading symbols from dmmc-stamp-breakout/dmmc-stamp-breakout.elf...
(gdb) tar ext :3333
Remote debugging using :3333
Reply contains invalid hex digit 120
(gdb) 

$ pyocd gdbserver
...
0012986:DEBUG:packet_io:--<<<< GDB send 101 bytes: b'$l<?xml version="1.0"?><!DOCTYPE feature SYSTEM "threads.dtd"><threads><thread id="1" /></threads>#41'
0012987:DEBUG:packet_io:-->>>> GDB read 13 bytes: b'$qAttached#8f'
0012987:DEBUG:gdbserver:GDB received query: [b'Attached#8f']
0012987:DEBUG:packet_io:--<<<< GDB send 5 bytes: b'$1#31'
0012988:DEBUG:packet_io:-->>>> GDB read 8 bytes: b'$Hc-1#09'
0012988:DEBUG:packet_io:--<<<< GDB send 6 bytes: b'$OK#9a'
0012989:DEBUG:packet_io:-->>>> GDB read 12 bytes: b'$qOffsets#4b'
0012989:DEBUG:gdbserver:GDB received query: [b'Offsets#4b']
0012990:DEBUG:packet_io:--<<<< GDB send 23 bytes: b'$Text=0;Data=0;Bss=0#04'
0012991:DEBUG:packet_io:GDB packet thread: other side closed connection
0012991:DEBUG:packet_io:GDB packet thread stopping
0013090:INFO:gdbserver:Client disconnected!

Reproduce the connection failure with pyocd gdbserver and arm-none-eabi-gdb, using the attached log for the packet exchange. Start in pyocd/gdbserver/gdbserver.py around line 928 and trace the qOffsets response and packet handling. Done means GDB accepts the response and keeps the remote debugging connection open.