I had a similar issue on our dedicated server with Ionos back in 2018. I'm not sure if it will work for you, but the only way I found to solve it was by adding this to the existing CMSB php.ini file under /system:
session.gc_probability = 0
I also needed to make a new php.ini and add that into all of these DIRs:
\system\data
\system\lib\fieldtypes
\system\lib\menus
\system\lib
\system\plugins
\system\plugins\facebookLogin
\system\plugins\permalinks\languages
\system\plugins\permalinks
\system\plugins\permalinks\pluginSchemas
\system\plugins\twitterLogin
\system\plugins\websiteMembership
The issue only occurs with CMSB.