We have a question regarding cash collaterals that are held in other institutions as open position (on derivatives/loans) collaterals.

In general, should they be treated in Solvency II as cash and cash equivalents (balance sheet pos. R0410) or other receivables (R0380)?

Cash collaterals, by definition, cannot be freely disposed over (are not directly usable), this is why we think that receivables position (R0380) is more appropriate than R0410 for cash collaterals.