Manual:$wgSessionCacheType

Details
The cache type for storing session data. From MediaWiki 1.20 to 1.26, it is used only if   is true. From MediaWiki 1.27, this other configuration parameter is always considered as true.

For available types see  .

For proper operation this must be set to a persistent cache, and if there are multiple servers that might serve a single client's requests the cache must be shared by all of them.

If   is set to  and this is left at its default , the cache used may not meet these requirements.

The solution is to set this to an appropriate cache, such as. The caveat of this solution is that users cannot longer log in when the respective wiki's database was locked with e.g..