I too am having this issue- It's a fresh install, i have tried the install a couple of times, and the last time I deleted all files related to Observium, and cleared out the docker and started over with completely default settings. No luck so far.
I can add a device, but when i try to look at the device this is the message that I get. I also never get a confirmation that it was successfully added in the first place, however it also never fails. In my other installs of observium (not docker) it always gives a confirmation that the device was added successfully.
I can try to gather some logs and report back.
******* Update *******
Reading in previous posts, it was suggested to try deleting the entire Observium folder in appdata and restarting the docker- for whatever reason, that seemed to do the trick for me this time.
Starting up again after deleting the Observium folder in appdata, devices added properly and when device pages load normally. Not sure what went wrong.
The only thing I changed last time was a few values in the config.php value to add in Community strings and networks to scan... I don't think that my config.php changes would have broken anything though. However- i can't deny that blowing away the entire observium directory fixed everything for me. I have everything 100% default now.