I was able to reproduce this just now with only SpellTokens enabled. Note I do not get this at all on the DM side. This only happens with a second client (same PC connected to "localhost" or a second PC over LAN, same results), not on the DM client at all. It also doesn't matter if the NPC sheet is shared or not; to reproduce locally, I just connected as a player client and opened the first available NPC sheet from the PHB (Bat) and every time I mouse over "Melee Weapon attack" the error occurs.
Code:
[7/26/2021 3:05:00 PM] Client ruleset download complete
[7/26/2021 3:05:00 PM] Launcher scene exiting.
[7/26/2021 3:05:01 PM] Tabletop scene starting.
[7/26/2021 3:05:01 PM] NETWORK STATUS: [Client] [Connected]
[Client Type - DIRECT]
[7/26/2021 3:05:09 PM] MEASURE: RULESETS LOAD - 8.7379745 - 5E
[7/26/2021 3:05:09 PM] MEASURE: EXTENSIONS LOAD - 0.0180025 - 1
[7/26/2021 3:05:14 PM] MEASURE: MODULE LIST BUILD - 4.1619823 - 67
[7/26/2021 3:05:14 PM] MEASURE: REFRESH IMAGE ASSETS - 0
[7/26/2021 3:05:14 PM] MEASURE: REFRESH PORTRAIT ASSETS - 0.0070001
[7/26/2021 3:05:14 PM] MEASURE: REFRESH TOKEN ASSETS - 0.1110001
[7/26/2021 3:05:14 PM] MEASURE: ASSET LIST BUILD - 0.1180002
[7/26/2021 3:05:16 PM] MEASURE: LOAD - PART 1 - 15.3318128
[7/26/2021 3:05:17 PM] RULESET: Dungeons and Dragons (5E) ruleset (v2021-07-06) for Fantasy Grounds
Copyright 2021 Smiteworks USA, LLC
[7/26/2021 3:05:17 PM] RULESET: Core RPG ruleset (v2021-07-06) for Fantasy Grounds
Copyright 2021 Smiteworks USA, LLC
[7/26/2021 3:05:17 PM] EXTENSION: Spell Tokens v3.9.8\r\n
[7/26/2021 3:05:17 PM] MEASURE: LOAD - PART 2 - 1.3296638
[7/26/2021 3:05:18 PM] NETWORK SEND FILE REQUEST: campaign/images/arena.jpg
[7/26/2021 3:05:18 PM] NETWORK SEND FILE REQUEST: tokens/aarakocra.png@DD MM Monster Manual
[7/26/2021 3:05:18 PM] NETWORK SEND FILE REQUEST: tokens/abominableyeti.png@DD MM Monster Manual
[7/26/2021 3:05:18 PM] NETWORK RECV FILE REQUEST: campaign/images/arena.jpg
[7/26/2021 3:05:18 PM] NETWORK RECV FILE REQUEST: tokens/aarakocra.png@DD MM Monster Manual
[7/26/2021 3:05:18 PM] NETWORK RECV FILE REQUEST: tokens/abominableyeti.png@DD MM Monster Manual
[7/26/2021 3:05:18 PM] NETWORK SEND FILE REQUEST: campaign/portraits/id-00001
[7/26/2021 3:05:18 PM] NETWORK RECV FILE REQUEST: campaign/portraits/id-00001
[7/26/2021 3:05:18 PM] NETWORK RECV FILE REQUEST: campaign/portraits/id-00002
[7/26/2021 3:05:18 PM] NETWORK RECV FILE REQUEST: campaign/portraits/id-00003
[7/26/2021 3:05:18 PM] NETWORK SEND FILE REQUEST: campaign/portraits/id-00002
[7/26/2021 3:05:18 PM] NETWORK SEND FILE REQUEST: campaign/portraits/id-00003
[7/26/2021 3:05:23 PM] MEASURE: MODULE LOAD - 4.9639969 - D&D Player's Handbook
[7/26/2021 3:05:24 PM] NETWORK RECV FILE REQUEST: campaign/portraits/id-00004
[7/26/2021 3:05:24 PM] NETWORK SEND FILE REQUEST: campaign/portraits/id-00004
[7/26/2021 3:05:26 PM] MEASURE: MODULE LOAD - 2.4330007 - D&D Tasha's Cauldron of Everything - Players
[7/26/2021 3:05:30 PM] MEASURE: MODULE LOAD - 3.5919999 - D&D Xanathar's Guide to Everything - Players
[7/26/2021 3:06:27 PM] NETWORK SEND FILE REQUEST: tokens/B.png@DD PHB Deluxe
[7/26/2021 3:06:27 PM] NETWORK RECV FILE REQUEST: tokens/B.png@DD PHB Deluxe
[7/26/2021 3:06:28 PM] [ERROR] Script execution error: [string "scripts/SpellTokenList_sts.lua"]:120: setValue: Unable to create node