Core v1.0: Flashing from Web IDE no longer possible after Factory Reset

Last week I did some major updates on 2 Cores from Web IDE, everything worked well (I choosed Device OS target 0.7.0 because compiling with 1.1.0 ended up with an error), all of them are 1st generation (end of 2013, white circuit board). Yesterday I tried to do some minor changes in the code and during flashing both cores hung up, a second try to flash them failed. So I did a factory reset, did the WiFi settings via Serial and the cores started breathing cyan. The cores were also visible in Particle Console, “Last Handshake” updates immediately after pressing reset button, but if I try to flash them nothing happens, the core is still breathing cyan. Verify for my code said Ok. Tried flashing “Blink an LED”, also no success. Any idea how to solve this issue?

Try flashing via USB