Can't connect to router

i’ve flashed 0.12…1-b1 (and also tried 0.12.0) - i entered my Wifi credentials properly (several times) but my Wemos D1 won’t connect to my router.
The router is showing the D1’s Mac address under “Unsuccessful login attempts”.
I already tried different settings (wifi channel 1+11, changing WPA …) and did a factory reset with the router, no success …
The strange thing is: it worked for a short period of time but when dis- and reconnecting it can’t connect anymore.
any ideas what could be wrong?

Update: i flashed another fresh device - same issues … its MAC address lists under failed login attempts :confused:

PS: all other instances - they are also the same Wemos D1 - are connecting without problems

Network must be 2.4GHz, network name without spaces.