Solution : https://service.sap.com/sap/support/notes/863907 (SAP Service marketplace login required)
Key words :
adjust material groupww015 ww 015 ww406 ww 406 wg21 wg22 wg23 wg24 wgtr reason, key fields -> define material groups workaround, logistics - general -> material master -> settings, ww 406 - class type xyz, system issues error message ww015, material group maintenance activity, internal class type '026', call transaction wg22, maintaining material groups, maintain material groups
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| 1227242 | FAQ: Differences between material master & article master |
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| 359940 | FAQ: How do I maintain material groups? |