At the end I succeeded recovering my DSO Quad !
You’ll find attached to this post the stock DSO Quad Bootloader v3.10; it has to be downloaded at the embedded Flash base address that’s 0x80000000.
Once restored the stock bootloader you will be asked for a License number that is calculated starting from your Hardware Serial number. In order to know it you can use the SYS FW 2.50 I modified to show you at startup the License number further to the Serial number.
So take care to note it down before doing any FW update that could potentially corrupt the original bootloader. 
When you have taken note of your License number you can finally feel free to do any sort of experiment with your DSO Quad since you will be able to restore it completely at ANY time ! 
Hope it helps.
SYS250a.zip (26.1 KB)
BootLoader v3.10.zip (8.79 KB)