I built task with -DCMAKE_BUILD_TYPE=debug, then run it with gdb. The " (4)" in the final error message below is my annotation to find the correct source line.
$ stty size
67 110
$ gdb ./src/task
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Reading symbols from ./src/task...done.
(gdb) set args all
(gdb) catch throw
Catchpoint 1 (throw)
(gdb) run
Starting program: /home/russells/src/tasktools/task/src/task all
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Catchpoint 1 (exception thrown), 0x00007ffff754826d in __cxa_throw ()
from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
(gdb) bt
#0 0x00007ffff754826d in __cxa_throw () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#1 0x00007ffff75487ec in operator new(unsigned long) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#2 0x00007ffff75d6ae9 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_mutate(unsigned long, unsigned long, char const*, unsigned long) ()
from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#3 0x00007ffff75d8073 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_append(char const*, unsigned long) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#4 0x0000555555763009 in Color::_colorize (this=0x7fffffffbf20,
result="\033[1;37m 2017/06/18 Xxxxxxxx xxxxxx. ",
input=" 2017/06/18 Xxxxxxxx xxxxxx. ")
at /home/russells/src/tasktools/task/src/libshared/src/Color.cpp:498
#5 0x0000555555762c62 in Color::colorize (this=0x7fffffffbf20,
input=" 2017/06/18 Xxxxxxxx xxxxxx. ")
at /home/russells/src/tasktools/task/src/libshared/src/Color.cpp:408
#6 0x000055555574f40f in Column::renderStringLeft (this=0x555555f04000,
lines=std::vector of length 2, capacity 2 = {...}, width=39, color=...,
value=" 2017/06/18 Xxxxxxxx xxxxxx.", ' ' <repeats 16 times>)
at /home/russells/src/tasktools/task/src/columns/Column.cpp:323
#7 0x00005555557529a1 in ColumnDescription::render (this=0x555555f04000,
lines=std::vector of length 2, capacity 2 = {...}, task=..., width=39, color=...)
at /home/russells/src/tasktools/task/src/columns/ColDescription.cpp:183
#8 0x000055555578e0f4 in ViewTask::render[abi:cxx11](std::vector<Task, std::allocator<Task> >&, std::vector<int, std::allocator<int> >&) (this=0x7fffffffc660, data=std::vector of length 1694, capacity 2048 = {...},
sequence=std::vector of length 1694, capacity 2048 = {...})
at /home/russells/src/tasktools/task/src/ViewTask.cpp:316
#9 0x00005555556ef1c3 in CmdCustom::execute (this=0x555555a4a670, output="")
at /home/russells/src/tasktools/task/src/commands/CmdCustom.cpp:208
#10 0x00005555556388e3 in Context::dispatch (this=0x7fffffffd4e0, out="")
at /home/russells/src/tasktools/task/src/Context.cpp:825
#11 0x000055555563790e in Context::run (this=0x7fffffffd4e0)
at /home/russells/src/tasktools/task/src/Context.cpp:675
#12 0x0000555555633531 in main (argc=2, argv=0x7fffffffe0a8)
at /home/russells/src/tasktools/task/src/main.cpp:52
(gdb) cont
Continuing.
Unknown error. Please report. (4)
[Inferior 1 (process 4938) exited with code 03]
(gdb)