Hello Pasi
BoxCryptor for Android also supports self-signed certificates. So make sure your certificate is encoded as .crt or .pem file. Also make sure you store it on your sdcard or internal storage and don't remove it from the location you have set up.
Also make sure the file only contains your certificate. Here is an example:
-----BEGIN CERTIFICATE-----
MIIDaRFdfdvCBFDsGscwDQYJKoZIhvcNAQEFBQAweTEYMBYGCSqGSIb3DQEJARYJ
YmNAYmMuY29tMQswCQYDVQQGEwJERTEMMAoGA1UECAwDYmM0MQwwCgYDVQQHDANi
YzMxDDAKBgNVBAoMA2JjMjEMMAoGA1UECwwDYm+zRRgwFgYDVQQDDA8xOTIuMTY4
LjE3OC4xMTQwHhcNMTMwMTA4MThrhtrhhcNMTQwMTA4MTMFxMDMxWjB5MRgwFgYJ
KoZIhvcNAQkBFgliY0BiYy5jb20xCzAJBgNVBAYTAkRFMQwwCgYDVQQIDANiYzQx
DDAKBgNVBAcMA2JjMzEMMAoGA1UECgwDYmMyMQwwCgYDVQQLDANiYzExGDAWBgNV
BAMMDzE5Mi4xNjguMTc4LjExNDCCASIwDQYfgdfghvcNAQEBBQADggEPADCCAQoC
ggEBAIJ1r99JedniM2asNrSNaDNDIavRlZSarnMKLNXBqyYomO00WpZJJeinJYUk
mf03iodnXuqaW0MEkGKcOgSFXJvxGDAKxMvEXYU9CG8M3fPSQ5xTadWP36oULi7R
KQ2wKKRwC9giO50+JeuSGIhxo9QM9KKW39Hif5JayCQEZsDP14hoc9LYTgrZ22ra
5iH0Upa+OoO6ohgm/fzcJ6djkobfPAmon+TwmXpI5HugYI7cfflqDefUKgG7MOls
AFWyw/bDktUtFWw/X8ysvxOyFx/3M5+qBp8JV9iIj+C0qP8VohsiBhhUIMyUKvYq
DvywpoVSNVj4q1XZU+2FLpDe6F8CAwEAATANBgkqhkiG9w0BAQUFAAOCAQEAROFL
XZCAnCMsfgzfnbHYG+1twfOyBHe6tNvLpLQBiNM0Uiw35H7tQkeuXRK0RSgqNtOb
PzqhVLW9XVbtFvFnGtUugK/HdfoqgqSkfcZwMtGPoC+H38+QlA+eScVqGljfOeZV
hYjz/sBxfkiZzHY2XP3l476USuHd1PcV4dUFZoQxOX3jnQMtZ7CQsrppJzYVCmU+
njOKNH4/Rj8Nn6a+YuBDR8QlF6ffxAfh54z4ftgMzukX/SlIsn7Ia+vaTXkAbQZA
RBKTNu4iBF6ZJLmsgcCor4i0aF8iAdhtZngYvjgWXTSzlxkiCwRV/kW9gLgcANu3
u9VMexSGZBETYZf0CA==
-----END CERTIFICATE-----
Best regards
Boris
