Within Europe, whenever there is need to reduce the number of flights through a certain airspace, the Network Manager (Eurocontrol) puts in a so called "regulation". This results in flights planned through the regulated airspace being delayed or re-routed.
Regulations are given an ID; those starting with KALP are from the Karlsruhe Upper Area Control (EDUU), southern sectors (Alps) above Flight Level 315.
Since your flight was over three weeks ago, I can't check the exact "regulation Id" in the public database, but it is likely that it was related to staffing issues at Karlsruhe upper area control centre.
An example of a current regulation starting with "KALP" is below:
Karlsruhe UAC has severe staffing problems and is causing many traffic delays this year.
A delay of 6 hours due to a regulation sounds extreme to me. I don't know what the origin and destination of the flight was, but typically a re-routing avoids the regulated airspace with only a short delay.