Update – may have found a possible solution:
After reporting an issue (libgomp failing to create a thread) to my ISP, they did some testing and added the following to my root .htaccess file:
RLimitNPROC max max
RlimitMEM Max
RLimitCPU Max
At the time ReSmush.it was deactivated but Postie was failing to complete processing (running every 2 hours over an extended period of about 24 hours) and resulting in the following messages in the error log (not all shown for the 24 hour period):
rosamundewood.art [Tue Aug 10 15:40:04 2021] [error] [client 45.8.227.175:0] libgomp: Thread creation failed: Resource temporarily unavailable
rosamundewood.art [Tue Aug 10 17:40:17 2021] [error] [client 45.8.227.174:0]
rosamundewood.art [Tue Aug 10 17:40:17 2021] [error] [client 45.8.227.174:0] libgomp: Thread creation failed: Resource temporarily unavailable
rosamundewood.art [Tue Aug 10 19:40:17 2021] [error] [client 45.8.227.174:0] Postie [error]: fetch_mail: Unable to write data to IMAP server imap.stackmail.com on port 993
rosamundewood.art [Tue Aug 10 21:44:04 2021] [error] [client 45.8.227.174:0]
rosamundewood.art [Tue Aug 10 21:44:04 2021] [error] [client 45.8.227.174:0] libgomp: Thread creation failed: Resource temporarily unavailable
rosamundewood.art [Tue Aug 10 23:43:57 2021] [error] [client 45.8.227.174:0]
rosamundewood.art [Tue Aug 10 23:43:57 2021] [error] [client 45.8.227.174:0] libgomp: Thread creation failed: Resource temporarily unavailable
rosamundewood.art [Wed Aug 11 01:44:15 2021] [error] [client 45.8.227.175:0]
rosamundewood.art [Wed Aug 11 01:44:15 2021] [error] [client 45.8.227.175:0] libgomp: Thread creation failed: Resource temporarily unavailable
Also, on each failed run, the correct image was loaded into the image library and a blank draft post (tmptitle) created.
After the amendment of the .htaccess file, this seemed to cure the problem. Have just retried a couple of posts with ReSmush.it activated, and not had any issues.