FTPS - Unsupported or unrecognized SSL message

I'm trying to set up a FTPS connection, not to confuse with SFTP. For this I'm using the publicly available module SFTPS module (link). Regardless of my actions, it keeps returning the error message “Unsupported or unrecognized SSL message" (bit longer stacktrace at bottom). Who knows the fix or has a suggestion for a next step? For testing purposes I'm using test.rebex.net, as the actual connection has security to be setup with a whitelist, only accessible from the Mendix cloud servers. FileZilla has no issues with Rebex, but Mendix has. Both FTP and SFTP is succesfull, but using FTPS is not. Switching over to SFTP is no option, we have to stick with FTPS. I've tested with both Mendix 9.12.4 (current app version) as well as a brand new 8.18.15 release (latest LTS near release of SFTPS module). Changing Mendix versions makes no difference. After googling for a bit, a lot of results referenced the libraries, but updating those also brings no avail. The clean 8.18.15 version, after installing the module, contains only 3 libraries in the Userlib folder. I've updated all three, without a difference. To name ‘em: jsch (0.1.55), commons-io(2.11.0) and commons-net(3.8.0).  Who has the solution or can suggest a step to try?   javax.net.ssl.SSLException: Unsupported or unrecognized SSL message at java.base/sun.security.ssl.SSLSocketInputRecord.handleUnknownRecord(SSLSocketInputRecord.java:439) at java.base/sun.security.ssl.SSLSocketInputRecord.decode(SSLSocketInputRecord.java:184) at java.base/sun.security.ssl.SSLTransport.decode(SSLTransport.java:108) at java.base/sun.security.ssl.SSLSocketImpl.decode(SSLSocketImpl.java:1152) at java.base/sun.security.ssl.SSLSocketImpl.readHandshakeRecord(SSLSocketImpl.java:1063) at java.base/sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:402) at org.apache.commons.net.ftp.FTPSClient.sslNegotiation(FTPSClient.java:283) at org.apache.commons.net.ftp.FTPSClient._connectAction_(FTPSClient.java:219) at org.apache.commons.net.SocketClient._connect(SocketClient.java:254) at org.apache.commons.net.SocketClient.connect(SocketClient.java:212) at sftps.helpers.FTPSConnection.execute(FTPSConnection.java:77) at sftps.actions.JA_GetFileList.executeAction(JA_GetFileList.java:63) at sftps.actions.JA_GetFileList.executeAction(JA_GetFileList.java:29)  
1 answers

I believe that error message is usually when the Java code Mendix is using is expecting encrypted content back, but is instead receiving unencrypted content from the server. 

Can you check if the FTPS server is definitely returning encrypted content?