I reinstalled splunk to a different volume and now I get this message when trying to search for any string. How can I fix this ?
This message is telling you the search is a zombie.
The specific line of code that makes this determination in splunkd is:
bool isZombie = !(isDone || ScopedAliveProcessToken::isAlive(ddir / ALIVE_TOKEN));
Ie, if it is not done, and not still alive, it is a zombie. In other words, this is a search that crashed (or somehow otherwise stopped talking to the outside world), but still left some stuff lying around (which will be cleaned up later). So the search artifacts exist in some state, and you can identify that the job once existed, but it probably isn't useful for information for making IT decisions etc about.
The token is typically a named pipe, so you could have a case where the named pipe failed to be created or opened (permissions, fd exhaustion) but most likely there was a crash.
Check the var/log/splunk directory for crashlogs, and if you're getting them, create a diag and work with support. (both of these are talked about here)
and eventually create a support case.