According to other sources (German: https://muenchen.t-online.de/region/muenchen/id_100983050/mu...) some experienced surfers also say that the water level has been unusually low since the reopening (1.40 m and has since dropped to 1.21 m instead of the usual 1.50 m), which of course influences the flow. So some issue with the weirs regulating the flow is suspected - if it's "only" this, it might be quick to fix...