mobdebug suddenly gets out of sync

[1linux]

In a project (which relies heavily on copas an coroutines) mobdebug suddenly shows these lines, however keeps on working - but verly slow as these lines are repeated permanently.
How can I track down this bug?

INFO 2019-09-13 09:30:40 ...Public\Lua\ZeroBraneStudio\lualibs/mobdebug/mobdebug.lua:187: attempt to call field '__tostring' (a nil value)
stack traceback:
c:\lua\systree\share\lua\5.1\coxpcall.lua:35: in function 'pcall'
...Public\Lua\ZeroBraneStudio\lualibs/mobdebug/mobdebug.lua:187: in function 'val2str'
...Public\Lua\ZeroBraneStudio\lualibs/mobdebug/mobdebug.lua:224: in function 'val2str'
...Public\Lua\ZeroBraneStudio\lualibs/mobdebug/mobdebug.lua:248: in function <...Public\Lua\ZeroBraneStudio\lualibs/mobdebug/mobdebug.lua:147>
INFO 2019-09-13 09:30:40 ...Public\Lua\ZeroBraneStudio\lualibs/mobdebug/mobdebug.lua:188: attempt to call field '__serialize' (a nil value)
stack traceback:
c:\lua\systree\share\lua\5.1\coxpcall.lua:35: in function 'pcall'
...Public\Lua\ZeroBraneStudio\lualibs/mobdebug/mobdebug.lua:188: in function 'val2str'
...Public\Lua\ZeroBraneStudio\lualibs/mobdebug/mobdebug.lua:224: in function 'val2str'
...Public\Lua\ZeroBraneStudio\lualibs/mobdebug/mobdebug.lua:248: in function <...Public\Lua\ZeroBraneStudio\lualibs/mobdebug/mobdebug.lua:147>
Debugging session completed (traced 1 instruction).

[pkulchenko]

@1linux, this doesn't look like mobdebug is getting out of sync; this is a case of an error inside pcall being reported by your interpreter for some reason. Are you using a custom version of pcall? These errors shouldn't be reportable at all.

[pkulchenko]

Closing. Please reopen if there is new information on this issue.