In the following composition a white solid mass arises. How to solve it? Erythromycin 2%, Isopropanol 40 ° ad 200ml

Referring to the composition included in the TMF I would suggest to bring the isopropanol content at 50, possibly 60% (M/m). You can increase the content by 10% in order to prolong the retention duration and add 200mg of anhydrous citric acid to set the ideal pH.