I assume that your CDB$ROOT is open.
So, did you try to resize your CDB$ROOT (by shrinking one more datafiles)?
This could help you to come below the limit so that you can open your PDB.
I think the first things you want to do is to check the actual alert.log, which should be located at:
Check for clues.
Next, before you make any changes, I suggest to perform a full RMAN backup.
Perhaps you can reclaim some space from the temp datafile, though I'm not sure if the space counts toward the 12GB limit. But perhaps worth a try:
SQL> ALTER TABLESPACE TEMP SHRINK SPACE;
I run into the same. It's a very ugly error.
My only suggestion is to install an Oracle Standard Edition Home and try to run/append the datafiles of XE from the new home, then shrink the tables and export/import back into XE.
Delete the auxiliary dbhome.
Same issue in my testlab.
My XEPDB1 was unable to "open read write" or "open read only" (ora-12954). No chance to cleanup the user tablespace in the mounted XEPDB1. The only way was to append the XE datafiles in a "auxiliary" Oracle 18c home. There I could start this XE. Now I could see which objects filled up the user tablespace. In my case there was the user tablespace mostly filled by fulltext search indexes. The indexes were regular synchronized by a scheduler job with "ctx_ddl.sync_index". I assume during this user maintenance jobs with ctx_ddl package the 12GB space limit will not correctly checked and an overfill will sadly allowed. I think it's a XE bug.
However now I could drop the fulltext indexes. Export schema, recreate the tablespace with compress (this is allowed in XE) and import back.
After appending datafiles back to XE at last I could start the XE instance without problems.
I am having the same issue , but i am unable to understand the procedure for "append the XE datafiles in a "auxiliary" Oracle 18c home" i tried to take users tablespace dbf file to another system with a clean xe database install and added this dbf to previously present USERS tablespace in newly installed XE database but i cannot see any content from the tablespace
Can you kindly provide some detail on how to perform this procedure , also is it needs to be imported into home database or can it work in a fresh xe install like i am doing?