Arcbit is now an inactive wallet. There have been no PRs, issues or any sort of development happening over in github page and also the website's certificate has not been renewed. You should probably leave the wallet aside and move onto Bitcoin Core/Electrum as mjglqw suggested.
Since you have the 12 word passphrase, try importing the words in a trusted wallet in Electrum. You would be able to access your funds there. Do the following to access your funds in electrum.
1. Download Electrum from official website. Verify the PGP key of ThomasV before entering your passphrase.
2. Select 'Standard Wallet' and click on Next
3. Select 'I already have a seed' and click on Next
4. Enter the 12 word passphrase in the electrum window
Note that Arcbit is a legacy wallet as per the information from
trezor5. After entering the 12 word passphrase click on 'Options' instead of Next
6. Select 'BIP39 Seed' and select legacy(p2pkh) and enter derivation path as m/44'/0'/0' and click on Next
7. Enter the password and confirm it and then click on Next
You should be able to spend your coins there. Hope this might solve your problem.