
Problem solved ... with a little reverse engineering: Fortunately the phototransistors are quite unique and easy to identify as OSRAM SFH 325 FA-3/4-Z series, peaking at 980nm wavelength, and that gives a clue about what kind of LED is being used in the input alert circuit. For reference if anybody ever comes across the same issue:
The Diode DS2 is an infrared diode. Replacement with an SMD 0805 sized IR diode with 940nm wavelength seems to work just fine. In my case a Harvatek HT-170IRAJ was used. Just be careful that the height of the led is not to great, otherwise the diffusor in the multimeter case will crush your led when the case is reassembled
