Kleinkläranlage regulations worry

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(Kleinkläranlage are small bio water treatment plants. Ours has 5 different chambers.)

Ours came with the house that was built in 1997.
On enquiring about another issue re. its pump, I was told we should have a contract for yearly inspection and maintenance. (Not with this company, so they weren't trying to sell us a contract.) We could apparently be in trouble for not doing this. I'm sure when we registered with the local Wasserverband that I disclosed we had such a system, but they didn't give us any official directions what to do, as far as I remember, and I can't find any documentation amongst my water paperwork.
We're perfectly happy to get such a contract but now I'm worrying about contacting them and opening up the possibility of hefty fines for having not done this previously.
Would be grateful for any input (or reassurance!) Our village neighbours all just have simple cesspits so are clueless.