UNDO goes back only one step

Lately I have a problem with the UNDO function. Since we installed the newest V18, UNDO goes only back one step. I looked into the system variables, but this doensn't solve the problem.
I use LSP-files and I think there might be a problem... In all my routines I use (vl-cmdf "UNDO" "BEGIN") and at the end (vl-cmdf "INDO" "END")...

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