Disclosed are storage stable aqueous infusible or injectable solutions containing an active substance of general formula (I) wherein the groups L, R1, R2, R3, R4 and R5 have the meanings given in the claims and in the specification, and an amount of a physiologically acceptable acid or mixture of acids sufficient to dissolve the active substance and act as a stabilizer, optionally together with other formulating excipients suitable for parenteral administration, and a process for preparing the infusible or injectable solutions according to the invention.