Found the cause of this. From the mon.a.log:
5> 2018-03-14 16:55:26.470 7f61aa02a700 10 log_client will send 2018-03-14 16:55:26.473781 mon.a mon.0 192.168.178.29:40706/0 93 : audit [INF] from='client.154151 -' entity='client.admin' cmd=[{"profile": ["k=3", "m=3"], "prefix": "osd erasure-code-profile set", "name": "myprofile"}]: dispatch
... snip ...
/home/sebastian/Repos/ceph/src/mon/OSDMonitor.cc: 5501: FAILED assert((*erasure_code_profile_map).count("plugin"))
ceph version 13.0.1-3034-g619d435a71 (619d435a71f002571bda0c71dd26d75deaf480c3) mimic (dev)
1: (ceph::__ceph_assert_fail(char const*, char const*, int, char const*)+0x137) [0x7f61b722c9fb]
2: (OSDMonitor::parse_erasure_code_profile(std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const&, std::map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >*, std::ostream*)+0x145) [0x5584109774ef]
3: (OSDMonitor::prepare_command_impl(boost::intrusive_ptr<MonOpRequest>, std::map<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, boost::variant<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long, double, std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, std::vector<long, std::allocator<long> >, std::vector<double, std::allocator<double> > >, std::less<void>, std::allocator<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, boost::variant<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool, long, double, std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, std::vector<