Hello Dmytro.
This might be happening because the Process Portal Search Index is out of sync. I cannot confirm, but if users are seeing the process instance in a list that is generated by querying the search index, like the Processes instance list, and then try to click a process instance that doesn't exist, then this error might occur.
You can confirm if your Process Portal search index is out of sync by making use of the Index Diagnostic tools and using the information provided in this article I published: Process Portal Search Index Comprehensive. See the section titled, "How to determine an issue exists." The Index Diagnostic tool can be downloaded from that link too.
If you can confirm the Process Portal search index is out of sync, then you can try to complete the steps documented here to resolve: https://www.ibm.com/support/pages/what-first-step-resolving-problems-related-process-portal-search-index
If you still have issues after that, then you can open a case with IBM Support if you are running on a supported version of the product, or have a support extension in place for an out-of-support version.
Thanks and regards,
Seth Merriss
IBM Business Automation Workflow Support team
#BusinessProcessManager(BPM)#Support#SupportMigration