If you still receive the error in the most current version, try adjusting the Split options in iExplorer preferences on the Data Exporting tab and try the export again. The Small split setting will have the best performance for the really large conversations.
If you still get the error after adjusting the Split options, please try closing iExplorer and reopening and then go straight to the Messages section and try exporting just a single text message with that error to PDF. We’ve found that by closing and reopening iExplorer immediately before and after each PDF export, it fixes that error. If you have a lot of text messages with this error, you could try exporting all instead by clicking Export All > "Export all conversations as PDF files...".
If this is a really long conversation or one with a lot of image attachments (which it usually is for this error), you also have the option to export the conversation in 2-3 date segments by adjusting the date range for the conversation and exporting each date range to PDF. Above a conversation view, you’ll see a beginning and end date for the conversation. You could first try picking a date somewhere in the middle and try setting the end date to it for just the first half of the conversation. Then, once the first half is in view, you can export it to PDF after selecting all the bubbles. To do this, just select any of the text bubbles and then press Control-A to select all bubbles in that date range and click on the Export Selected button above the conversation on the right and choose PDF.
If the first half of the conversation date range works, then you can adjust the date range for the conversation again in iExplorer and export the 2nd, more recent half. If you still got the same error for the first half, please try a smaller date range and try again. You should definitely be able to ultimately get all of your text messages exported to PDF without breaking it into more than 3 or so date range batches.
Then, if you need to combine the PDFs into one after the transfer, we actually have an online PDF editor web app that can combine PDFs for free at https://dochub.com. You can create a DocHub account automatically with your Google or Dropbox account or create one with any email and password.
If this error is happening in version 184.108.40.206 or later after trying everything above, please submit a support ticket and let us know if you are able to export these same text message(s) as TXT or CSV files and that the error still happened when Split was set to Small, and whether you tried the multiple date range segments option.