Trial keys are not supposed to work with old trial versions. When evaluating a Rebex product, please use the latest version that is supported and compatible with contemporary servers.
When you installed the trial package of Rebex SFTP 2020 R5, you accepted Rebex General License Agreement which grants you a permission to install the trial for evaluation purposes for a period of 30 calendar days from the date of installation.
When your 30-day evaluation period was over, and you should have either:
a. Purchase a license.
b. Uninstall the Rebex product and stop using the Rebex API.
c. Contact us at firstname.lastname@example.org and ask for the 30-day evaluation period to be extended.
For details, see What are Trial license limitations and the "THIRTY (30) DAY EVALUATION (TRIAL) USE LICENSE" section of Rebex General License Agreement.
If you have been using a trial version for many months, you have been in violation of the license agreement. Essentially, this is akin to continuously using Microsoft Windows in 30-day non-activated mode without purchasing a license. This is not allowed, and it is not supposed to keep working indefinitely.
On the other hand, licensed keys are suitable for all versions of Rebex SFTP released before the end of the licensed key's support contract, and those will keep working indefinitely with the same key. Licensed keys only have to be upgraded when a support contract is renewed.