Since it is for more than one user, check a few more to see if it is for ''all'' users.
If it is only for a few users (instead of all) , consider going into Site Admin and delete the ''trouble'' user account and recreate it, and then reassign it to all the projects you/they previously accessed (with appropriate group associations permissions). -- sometimes a user will get ''corrupted'' somehow in the database.
If it is for many or ''all'' users:
Confirm that it is happening for only one (or a few) databases vs. ALL databases/schemas.
All project schemas/databases, might point to the database server/instance or network connectivity having issues.
Look to see if the DRIVE containing the project repository has ample free space. If on Linux, look to see if the ''quota'' is close to macing out. -- Sometimes, when the repository drive is out of space, QC fails with odd errors because it cannot store any more artifacts relating to the project.
In the Database server, have the DBA look to see if the actual database files (under the database, e.g. in MS-SQL all databases have an LDB and MDB file stored somewhere) are being stored on a DRIVE (or in Oracle ''tablespace'') with ample space. -- Sometimes when the database drive is running out of room, QC will throw odd creation errors since new records can't be stored.
Also, ask the DBA is there is any maintenance happening related to this project's schema/database. this will take one or more database/schemas ''offline'' seemingly sporadically.