After issuing a sequence of ALTER TABLE commands Galera crashed completely on both nodes:

SQL Commands:

alter table table1 convert to character set utf8mb4 collate utf8mb4_unicode_ci;
alter table table2 convert to character set utf8mb4 collate utf8mb4_unicode_ci;
alter table table3 convert to character set utf8mb4 collate utf8mb4_unicode_ci;
alter table table4 convert to character set utf8mb4 collate utf8mb4_unicode_ci;
alter table table5 convert to character set utf8mb4 collate utf8mb4_unicode_ci;
alter table table6 convert to character set utf8mb4 collate utf8mb4_unicode_ci;
alter table table7 convert to character set utf8mb4 collate utf8mb4_unicode_ci;

mysql_error.log output/crash:

InnoDB: Assertion failure in file /home/buildbot/buildbot/padding_for_CPACK_RPM_BUILD_SOURCE_DIRS_PREFIX/mariadb-10.4.12/storage/innobase/data/data0type.cc line 67