WoW Classic, English client, EU server, happens on almost all chars.
Example is changing of a line or just switching between the config and tab-1 of (in this case) Sternferal, Druid Feral cat sequence. (author is Sternwind)

In this case.
Trying to remove the (rank 1) parts and changing a line.
(Also, Still not sure / clear if one better needs to use nill or null. Both give the same message when debugging though.)

Changing:
/cast [nostealth,nocombat] Prowl(Rank 1)
to
/cast [nostealth,nocombat] Prowl

/castsequence [nostealth,combat] Claw, Claw, Claw, Ferocious Bite(Rank 1)
to
/castsequence [nostealth,combat] Claw, Claw, Claw, Rake, Ferocious Bite

Error message:

Date: 2019-10-19 00:52:07
ID: 1
Error occured in: Global
Count: 1
Message: ..\AddOns\GSE\API\Translator.lua line 581:
attempt to concatenate a nil value
Debug:
(tail call): ?
GSE\API\Translator.lua:581:
GSE\API\Translator.lua:570
GSE\API\Translator.lua:597:
GSE\API\Translator.lua:589
GSE\API\Translator.lua:639: GetSpellId()
GSE\API\Translator.lua:199: TranslateSpell()
GSE\API\Translator.lua:118: TranslateString()
GSE\API\Translator.lua:18: TranslateSequence()
GSE_GUI\Editor.lua:663: GUIDrawMacroEditor()
GSE_GUI\Editor.lua:1061: GUISelectEditorTab()
GSE_GUI\Editor.lua:152:
GSE_GUI\Editor.lua:151
...

The complete export of the macro:

d0JTcaWsveVveBdL4WkntukpMQMTOURiDtvQ(fe9nuQ(he2ju2R0Ub2pf)eKQHrs(nQoVkPHscnyknCvIdcs50q1XuvNtrrlufLLsIwmKwUcpurjpvyzOyDkk1evrAQuXKPsMoYfvbUmXZiPUUQ0gvbTvfvSzvO2UIcFMk1Sa10uuvFxrLgPkK3POkJwvmovkNeL0Zj11aX9ur1Rjb3wrooi5(RtX(ncNduID4bkHgBfJPonC2SaiDdeWiGraJagbmcyenG2l9WhncsgG5IdC1qPSUGoiFTOnUVG3nZkGKXmeaHZbfTXPYX7BMk2VX91p4ZrgACoqxNI9RtX(nq6xnzSjaUMKntpPglegv)5zSjAs2m9KASSW(nifJPbs)QjTa1Ra8IXMa4As2m9a75tOlLASdrl050oClKXASF2vptvWglapNlM6gi9RM0cuVcWlgBcGRjzZ0dSNpHUuQXoeTqNt7WTqgRX(VXOg2yb45CXMFdK(vtAbQxb4fJnbW1KSz6b2ZNqxk1yHWO6dBSqyu9HnwimQ(WglddewU1Gno)cB4t4uJgodCGcc9lAJJK1vrBGjEfkAJZKmv0gkXbU)fTrqYLdFbEcNdAecuVcGW3CdNboqbHykAJtKRkLSSAPoLACICvXHO11Py)giPD4wibitxn2ea)zhtPjUK1vAXyAG0VAYyta8J5JPzRo1yzNLBftDdK(vtgBcGRjzZ0dmG75tOlLASqy(qk1qjoWntrBml(e6s9SgkoK7RFWRtPgkY1091p41PuPsnIlIN1tl2xL6gS65GgnosYxAuQula

STERNFERAL
Talents ?,?,?,?,?,?,?

This macro contains 1 macro version. This Sequence was exported from GSE 2.4.23.

Macro Version 1

Step Function: Sequential

KeyPress: Prowl(Rank 1), Cat Form

Main Sequence: Ravage, Ferocious Bite(Rank 1), Claw, Faerie Fire (Feral), Rake

This is a duplicate of #604 - Faerie Fire (Feral)

Until I get a Druid up high enough in classic I cant fix this.