In general, the solution may be a complete reinstallation of Java. 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 resolve the issue.

On Fedora, the following certificates are disabled for Java by default
​jdk.certpath.disabledAlgorithms=MD2, MD5, DSA, RSA keySize < 2048
​
On this line, the number 2048 needs to be replaced with 1024 (in the file /etc/crypto-policies/back-ends/java.config)
After making this change, 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

This 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