Yep, I would just try starting the server with a fresh world and see if it still happens.
If it doesn’t happen anymore, it is definitely a commandblock problem.
You can then disable command blocks in your server properties and then load up your world again and check for commandblocks.
Disable CommandBlocks in the server.properties: enable-command-block=false (This would disable every commandblock, and their functionality)
or type /gamerule commandBlockOutput false and /gamerule logAdminCommands false (This just makes the spamming stop, but the commandblock continues doing this silently)