In general, a complete reinstallation of Java may resolve the issue. Since the program now includes Java in its installation package, you need to uninstall and reinstall the application (download here).

Disabling SIP may also help. (Session Initiation Protocol)

If you are using Flexi in the cloud, installing the client package of the same version as your Flexi application may help.

In Fedora, the following certificates are disabled for Java by default
​jdk.certpath.disabledAlgorithms=MD2, MD5, DSA, RSA keySize < 2048
​
on this line, replace the number 2048 with 1024 (in the file /etc/crypto-policies/back-ends/java.config)
After that, Flexi will start up correctly.

Fedora 34 and openSUSE Tumbleweed are not yet officially supported operating systems. The verified solution is to edit the following line in the file /etc/crypto-policies/back-ends/java.config:
​jdk.certpath.disabledAlgorithms=MD2, SHA1, MD5, DSA, RSA keySize < 2048

Remove the SHA1 certificate.

The resulting line should look as follows:
​jdk.certpath.disabledAlgorithms=MD2, MD5, DSA, RSA keySize < 2048

For openSUSE Tumbleweed, the issue was also resolved by reinstalling Java to version 1_8_0-1.8.162-30 (from the polyconvex repository).

sudo update-crypto-policies --set LEGACY

The issue has also been observed with a Let's Encrypt certificate on CentOS 6. Solution:

1. Log in to PostgreSQL and delete the cscert table in centralServer

2. Restart Flexi — it will generate its own self-signed certificate

3. Upload the correct certificate