Rush android error saving to disc

HI

I have made a little video on my phone and it plays perfectly from with the Rush app (samsung s10)

WHen I export it I am told that it cannot save to the disc. I have tried several things:

• exporting a previous project on the same phone - works fine
• following instructions here - Error saving file to disc - which is about signing out, restarting and back in - no change
• going through selectively deleting everything track by track (made a copy!) - no difference
• a lot of searching on the internet

I opened the project on my wondows 10 desktop  version. Now I notice that two of the audio tracks I recorded are empty (but they play fine on Rush on my phone which suggests a sync issue????) - but phone and pc say they are synced

Please help!